UNITED STATES
                       SECURITIES AND EXCHANGE COMMISISION
                              WASHINGTON, DC 20549


                                    FORM 11-K




                   (X) ANNUAL REPORT PURSUANT TO SECTION 15(d)
                     OF THE SECURITIES EXCHANGE ACT OF 1934

                   FOR THE FISCAL YEAR ENDED DECEMBER 31, 2003

                                       OR

                 ( ) TRANSITION REPORT PURSUANT TO SECTION 15(d)
                     OF THE SECURITIES EXCHANGE ACT OF 1934
                        For the transition period from to
                              ---------------- ----

                         COMMISSION FILE NUMBER 1-14756




          A.   Full title of the plan and the address of the plan,  if different
               from that of the issuer named below:



                               AMEREN CORPORATION
                             SAVINGS INVESTMENT PLAN



          B.   Name of issuer of  securities  held  pursuant to the plan and the
               address of its principal executive office:


                               Ameren Corporation
                              1901 Chouteau Avenue
                            St. Louis, Missouri 63103











                Ameren Corporation
                Savings Investment Plan
                Financial Statements and Additional Information
                December 31, 2003 and 2002












Ameren Corporation
Savings Investment Plan
Index
December 31, 2003 and 2002
-------------------------------------------------------------------------------------------------------------------------
                                                                                                        

                                                                                                            Page(s)

Report of Independent Registered Public Accounting Firm...........................................................1

Financial Statements

Statements of Net Assets Available for Benefits...................................................................2

Statements of Changes in Net Assets Available for Benefits........................................................3

Notes to Financial Statements..................................................................................4-10

Additional Information*

Schedule I:       Schedule of Assets (Held at End of Year).......................................................11

Schedule II:      Schedule H, Line 4j - Schedule of Reportable Transactions......................................12



*    Other schedules required by 29 CFR 2520.103-10 of the Department of Labor's
     Rules and  Regulations  for  Reporting  and  Disclosure  under the Employee
     Retirement  Income  Security Act of 1974 ("ERISA"),  as amended,  have been
     omitted because they are not applicable.






             Report of Independent Registered Public Accounting Firm



To the Participants and Administrator of the
Ameren Corporation
Savings Investment Plan



In our opinion, the accompanying statements of net assets available for benefits
and the  related  statements  of changes in net assets  available  for  benefits
present fairly, in all material respects,  the net assets available for benefits
of the Ameren  Corporation  Savings Investment Plan (the "Plan") at December 31,
2003 and 2002,  and the changes in net assets  available  for  benefits  for the
years then ended in conformity with accounting  principles generally accepted in
the United States of America.  These financial statements are the responsibility
of the Plan's  management.  Our responsibility is to express an opinion on these
financial  statements  based on our  audits.  We  conducted  our audits of these
statements in accordance  with the  standards of the Public  Company  Accounting
Oversight  Board  (United  States),  which  require that we plan and perform the
audit to obtain reasonable  assurance about whether the financial statements are
free of material  misstatement.  An audit includes  examining,  on a test basis,
evidence  supporting the amounts and  disclosures  in the financial  statements,
assessing the  accounting  principles  used and  significant  estimates  made by
management,  and evaluating the overall  financial  statement  presentation.  We
believe that our audits provide a reasonable basis for our opinion.

Our audits  were  conducted  for the  purpose of forming an opinion on the basic
financial  statements  taken as a whole.  The  supplemental  schedules of assets
(held at end of year) and reportable  transactions are presented for the purpose
of  additional  analysis  and are not a  required  part of the  basic  financial
statements  but are  supplementary  information  required by the  Department  of
Labor's Rules and  Regulations  for Reporting and Disclosure  under the Employee
Retirement  Income Security Act of 1974.  These  supplemental  schedules are the
responsibility of the Plan's  management.  The supplemental  schedules have been
subjected  to the  auditing  procedures  applied  in  the  audits  of the  basic
financial  statements  and, in our  opinion,  are fairly  stated in all material
respects in relation to the basic financial statements taken as a whole.

The  schedule  of  assets  (held at end of year)  that  accompanies  the  Plan's
financial   statements   does  not  disclose  the  historical  cost  of  certain
nonparticipant-directed  Plan assets held by the Plan's  trustee.  Disclosure of
this  information is required by the Department of Labor's Rules and Regulations
for Reporting and Disclosure under the Employee  Retirement  Income Security Act
of 1974.



St. Louis, Missouri
June 18, 2004


                                       1






Ameren Corporation
Savings Investment Plan
Statements of Net Assets Available for Benefits
December 31, 2003 and 2002
------------------------------------------------------------------------------------------------------------------------

                                                                                      2003                 2002
                                                                                           
Assets
Investments (Note 3)                                                              $734,524,935         $626,996,779

Cash                                                                                         -                2,443

Receivables
    Participant contributions                                                          880,653            1,544,606
    Employer contributions                                                             313,236              626,752
    Dividends and interest                                                             216,663              477,104
                                                                            -------------------  -------------------

             Total receivables                                                       1,410,552            2,648,462
                                                                            -------------------  -------------------

             Total assets                                                          735,935,487          629,647,684

Liabilities
Accrued expenses                                                                        16,745               23,525
                                                                            -------------------  -------------------

             Net assets available for benefits                                    $735,918,742         $629,624,159
                                                                            ===================  ===================








   The accompanying notes are an integral part of these financial statements.

                                        2






Ameren Corporation
Savings Investment Plan
Statements of Changes in Net Assets Available for Benefits
Years Ended December 31, 2003 and 2002
------------------------------------------------------------------------------------------------------------------------
                                                                                     2003                 2002

                                                                                          
Investment income (loss)
Interest and dividends                                                           $ 20,350,203         $ 21,086,084
Net appreciation (depreciation) in fair value of investments                      106,136,129          (93,753,190)
                                                                            ------------------   ------------------

                                                                                  126,486,332          (72,667,106)
                                                                            ------------------   ------------------

Contributions
Participant contributions                                                          43,941,303           38,300,672
Employer contributions                                                             13,213,138           13,857,557
                                                                            ------------------   ------------------

                                                                                   57,154,441           52,158,229
                                                                            ------------------   ------------------

Plan transfer in (Note 1)                                                             512,250              600,509
                                                                            ------------------   ------------------

Benefits and expenses
Benefits paid to participants                                                      77,744,983           37,311,364
Administrative expenses                                                               113,457              129,041
                                                                            ------------------   ------------------

                                                                                   77,858,440           37,440,405
                                                                            ------------------   ------------------

             Net increase (decrease)                                              106,294,583          (57,348,773)

Net assets available for benefits
Beginning of year                                                                 629,624,159          686,972,932
                                                                            ------------------   ------------------

End of year                                                                     $ 735,918,742        $ 629,624,159
                                                                            ==================   ==================






   The accompanying notes are an integral part of these financial statements.

                                        3



Ameren Corporation
Savings Investment Plan
Notes to Financial Statements
December 31, 2003 and 2002
--------------------------------------------------------------------------------

1.   Description of the Plan

     General
     The  following is a brief  summary of the various  provisions of the Ameren
     Corporation  ("Ameren"  or the  "Company")  Savings  Investment  Plan  (the
     "Plan").  Participants  should refer to the Plan document for more complete
     information.

     The Plan's purpose is to provide certain  management and contract employees
     (the  "Participants")  of the Company and its wholly owned subsidiaries the
     option to defer a portion of their  annual  base  compensation  for federal
     income tax  purposes in  accordance  with  Section  401(k) of the  Internal
     Revenue Code (the  "Code").  The Plan is subject to certain  provisions  of
     ERISA,  as  amended,   and  regulations  of  the  Securities  and  Exchange
     Commission.

     The  Company  serves as sponsor  of the Plan,  and,  consequently,  has the
     authority to amend or terminate  the Plan subject to certain  restrictions.
     The Board of Directors of the Company has the authority and  responsibility
     for the general  administration of the Plan. The Northern Trust Company, as
     Trustee,  has the  authority  and  responsibility  to hold and  protect the
     assets of the Plan in  accordance  with Plan  provisions  and the  separate
     Trust Agreement.

     Effective  October 1, 2002,  the  Company  offered a  voluntary  retirement
     program to  approximately  1,000 of the Company's  7,400  employees.  To be
     eligible,  employees had to be age 50 or over, regular, full-time employees
     and have at  least 10 years of  service  with  Ameren.  In  December  2002,
     approximately 550 employees accepted the voluntary retirement program. Most
     of the employees who accepted left the Company by March 2003. Benefits paid
     to  Participants  on the  Statement of Changes in Net Assets  Available for
     Benefits for the year ended  December 31, 2003  includes  distributions  of
     Plan balances for certain of these employees.

     On January 31, 2003,  the Company  completed its  acquisition of all of the
     outstanding common stock of CILCORP Inc. from The AES Corporation  ("AES").
     With the  acquisition,  CILCORP Inc. became an Ameren  subsidiary.  Central
     Illinois Light Company is a wholly owned subsidiary of CILCORP Inc.

     Effective  January 1, 2004, the Plan was amended to merge the assets of the
     Employees'  Savings Plan of Central  Illinois  Light Company into the Plan.
     The assets transferred from the Employees' Savings Plan of Central Illinois
     Light  Company  consisted  of 29,767 and  931,778  shares of Ameren and AES
     common  stock,  respectively,  as of the date of the  transfer  with a fair
     market value of $1,369,268 and $8,795,987, respectively. In addition, other
     investments,  cash and receivables of $99,635,688,  and accrued expenses of
     $1,551 were transferred into the Plan.

     Certain   reclassifications  have  been  made  to  prior  year's  financial
     statements to conform to 2003 reporting.

     Participation
     The Plan covers  substantially  all  employees of the  Company,  except for
     employees of Central  Illinois  Light Company,  an indirectly  wholly owned
     subsidiary of the Company, prior to January 1, 2004, and contract employees
     covered by a  collective  bargaining  agreement  between  Central  Illinois
     Public Service Company  ("AmerenCIPS")  Local 702 IBEW or AmerenCIPS  Local
     148

                                        4



Ameren Corporation
Savings Investment Plan
Notes to Financial Statements
December 31, 2003 and 2002
--------------------------------------------------------------------------------

     IUOE and the  Company.  All regular  full time  employees  are  eligible to
     participate  upon  employment.   Participation  by  eligible  employees  is
     voluntary.

     Contributions
     Management participants may contribute from one to 50 percent of their base
     compensation to the Plan through payroll deductions.  Contract participants
     may  contribute  from one to 15 percent of their base  compensation  to the
     Plan  through  payroll  deductions.  Effective  July 1, 2003,  the  Company
     amended the Plan to permit contract  participants to contribute from one to
     100  percent  of  their  base  compensation  to the  Plan  through  payroll
     deductions.  Participant  contributions  are subject to annual  limitations
     imposed by the Code.  The  Company  will make an  Employer  Basic  Matching
     Contribution plus an Employer Additional Matching Contribution in an amount
     equal to a percent of the amount each Participant  contributes to the Plan,
     up to a certain maximum  percentage of the Participant's  compensation that
     he or she elects to contribute to the Plan each year. The amount of Company
     matching    contribution   depends   on   the   Participant's    employment
     classification  and for contract  employees is determined by the collective
     bargaining agreement with the specific union representing the Participants.
     A portion of  Company  matching  contributions  is  invested  in the Ameren
     Common  Stock  Fund.  Each  Participant's  portion  of  additional  Company
     matching contributions will remain invested in the Ameren Common Stock Fund
     until  Participants  reach  age 55. At age 55,  Participants  are given the
     opportunity to allocate their portion of Company matching  contributions to
     different  investments if so desired. All Company contributions are made to
     the extent  sufficient  earnings  are  available,  as described in the Plan
     document.

     Participants  direct  their basic  contributions  and the  Company's  basic
     matching  contributions by electing that such  contributions be placed in a
     single  investment  fund or allocated in  increments  of one percent to any
     combination  of investment  funds.  Such fund  allocation  elections may be
     changed daily.  Earnings derived from the assets of any investment fund are
     reinvested in the fund to which they relate.  Participants  may elect daily
     to  reallocate,  by actual dollar or percentage in one percent  increments,
     the value of their accounts between funds. Pending investment of the assets
     into  any  investment  fund,  the  Trustee  may  temporarily  make  certain
     short-term investments.

     Participant Loans
     The Plan  permits  Participants  to borrow from their  accounts  within the
     Plan. Such borrowings may be made subject to the following: (1) the minimum
     amount of the loan is $1,000, (2) the amount of the loan may not exceed the
     lesser  of  $50,000  or  fifty   percent  of  the  vested   amount  in  the
     Participant's  account,  (3) the loan will bear a fixed  interest  rate and
     repayments  will  be made  through  mutual  agreement  subject  to  certain
     statutory  repayment  time  limits,  (4) each loan shall bear a  reasonable
     interest rate as determined under policies established for the Plan and (5)
     such other  rules and  regulations  as may be adopted  by the  Company.  At
     December 31, 2003 and 2002, the interest rates on participant  loans ranged
     from 4.25  percent  to 11.62  percent  and 5.25  percent  to 10.5  percent,
     respectively.

     Vesting
     The amounts in Participants' accounts, including Company contributions, are
     fully vested at all times.

     Payment of Benefits
     The total amount of a  Participant's  account shall be  distributed  to the
     Participant  according  to one of the  options  as  described  in the  Plan
     document and as elected by the  Participant.  A  Participant  whose account
     balance is $5,000 or greater may defer  distribution  until  December 31 of
     the  year  they  attain  age 70 1/2 but no later  than  April 1 of the year
     following the Participant's attaining age


                                        5



Ameren Corporation
Savings Investment Plan
Notes to Financial Statements
December 31, 2003 and 2002
--------------------------------------------------------------------------------

     70 1/2. If the balance of the account is less than $5,000, the distribution
     shall be made in a lump sum within ninety days of his or her termination of
     employment,  provided  he or she  is not an  employee  on  such  date.  All
     distributions  shall be in the form of cash  except that  Participants  may
     elect to have his or her  interest  in the Ameren  Common  Stock  Fund,  if
     applicable,  distributed in shares of Ameren common stock. Participants may
     withdraw certain basic  contributions,  rollover  contributions and related
     earnings thereon upon reaching age 59 1/2, in the event of total disability
     or financial  hardship as defined by the Plan or the Code.  For purposes of
     distributions, the Participant's account value will be determined as of the
     last  business day  coincident  with or  immediately  preceding  the day of
     distribution.  Contributions to the Plan and investment  income thereon are
     taxable to Participants  upon  distribution  pursuant to the rules provided
     for under the Plan and the Code.

     The Plan also  provides,  to  participants  of the  former  Union  Electric
     Company  Employee Stock  Ownership  Plan and the former Ameren  Corporation
     Employee  Stock  Ownership  Plan for Certain  Employees  of CIPS and at the
     discretion  of the  Company,  for  distribution  prior  to  termination  of
     employment  of (a) all or a  portion  of a  Participant's  account  balance
     acquired at least 84 months prior to a distribution  and (b) any portion of
     a Participant's account balance acquired by dividends or other income.

     Plan Transfer In
     Plan transfers in represent  Participants' account balances which have been
     transferred during the year from the Ameren Corporation  Employee Long-Term
     Savings Plan - IUOE No. 148 and the Ameren  Corporation  Employee Long-Term
     Savings Plan - IBEW No. 702 into the Plan.

     Plan Termination
     The Company intends to continue the Plan indefinitely. However, the Company
     may at any time and for any reason,  subject to ERISA and Internal  Revenue
     Service  regulations,  suspend or  terminate  the Plan  provided  that such
     action  does  not   retroactively   adversely  affect  the  rights  of  any
     Participant under the Plan.

2.   Summary of Significant Accounting Policies

     Basis of Accounting
     The  accompanying  financial  statements  of the Plan are  prepared  on the
     accrual basis of accounting,  except that benefit  payments to Participants
     are recorded upon distribution.

     Use of Estimates
     The  preparation  of financial  statements  in conformity  with  accounting
     principles  generally  accepted  in the United  States of America  requires
     management  to make  estimates  and  assumptions  that affect the  reported
     amounts of assets and liabilities  and disclosure of contingent  assets and
     liabilities  at the  date of the  financial  statements  and  the  reported
     amounts  of  changes  in net  assets  available  for  benefits  during  the
     reporting period. Actual results could differ from those estimates.

     Investments
     All  investments  are  presented  at fair value as of December 31, 2003 and
     2002. The fair value of the Ameren Common Stock Fund was  determined  using
     year-end  published  market prices.  Investments  in equity  securities are
     valued at published net asset market value including  accrued

                                        6



Ameren Corporation
Savings Investment Plan
Notes to Financial Statements
December 31, 2003 and 2002
--------------------------------------------------------------------------------

     income on the last business day of each year.  Investments  in the Northern
     Trust Company's  Short-Term  Investment  Fund, the Northern Trust Company's
     Collective  Trust  Stable  Asset Fund,  and the T. Rowe Price  Stable Value
     Common  Trust  Fund  are  valued  at  cost  plus  accrued   income,   which
     approximates   market.   Participant   loans  are  valued  at  cost,  which
     approximates fair market value.

     Investment  securities are exposed to various risks, such as interest rate,
     market,  and  credit.  Due to the  level of risk  associated  with  certain
     investment  securities and the level of  uncertainty  related to changes in
     the value of investment securities, it is at least reasonably possible that
     changes  in risks in the near term  could  materially  affect  the  amounts
     reported in the Statement of Net Assets Available for Benefits.

     Income
     Interest  income is  recorded  on the  accrual  basis.  Dividend  income is
     recorded on the ex-dividend date.

     Gains and losses on security  transactions  are recorded on the trade date.
     Net unrealized  appreciation or  depreciation  for the year is reflected in
     net  appreciation  (depreciation)  in  fair  value  of  investments  on the
     Statement of Changes in Net Assets Available for Benefits.

     Administrative Expenses
     Trustee fees incurred in administering the Plan are charged to the Plan.









                                        7






Ameren Corporation
Savings Investment Plan
Notes to Financial Statements
December 31, 2003 and 2002
------------------------------------------------------------------------------------------------------------------------------------

3.   Investments

     The following  table presents  investments of the Plan at December 31, 2003 and 2002, respectively:


                                                                                             2003               2002
                                                                                                    
     Investments at Fair Value as Determined
     By Quoted Market Price

     Common Stock
         Ameren Corporation, $.01 par value(1)(2)                                       $ 186,997,506       $ 179,034,216

     Managed Equity Funds
         Lord Abbett Mid-Cap Value Fund(1)                                                113,828,229                   -
         Washington Mutual Investors Fund(1)                                               90,778,245          73,904,236
         Vanguard Asset Allocation Fund(1)                                                 68,985,586          55,730,317
         Barclays Global Investors Equity Index Fund(1)                                    48,503,305          35,725,072
         American Funds World Fund of America                                              33,873,792                   -
         MSIF Trust Fund                                                                            -          84,001,696
         Vanguard US Growth Fund                                                                    -          24,243,926
         Vanguard Extended Market Index Fund                                               24,257,077           6,650,011

     Managed International Equity Fund
         American Funds Europacific Growth Fund                                            28,383,017          18,820,977

     Managed Fixed Income Fund
         PIMCO Total Return Fund                                                           13,713,848          18,220,489

     Investments at Estimated Fair Value
     Managed Fixed Income Funds
         Northern Trust Company's Collective Trust Stable Asset Fund(1)                    83,122,275                   -
         T. Rowe Price Stable Value Common Trust Fund(1)                                   19,816,654         109,290,844

     Northern Trust Company's Collective Short-term Investment Fund                         4,537,265           3,904,650

     Participant Loans                                                                     17,728,136          17,470,345
                                                                                     -----------------  ------------------

                  Total investments                                                     $ 734,524,935       $ 626,996,779
                                                                                     =================  ==================



     (1)  Investments that represent 5 percent or more of the Plan's net assets.
     (2)  Nonparticipant-directed  portion is  $47,387,033,  and  $40,209,751 at
          December 31, 2003 and 2002, respectively.


                                       8





Ameren Corporation
Savings Investment Plan
Notes to Financial Statements
December 31, 2003 and 2002
------------------------------------------------------------------------------------------------------------------------------------


     During 2003 and 2002, the Plan's investments (including investments bought,
     sold,  and held  during  the year)  appreciated  (depreciated)  in value as
     follows:


                                                                                             2003                2002
                                                                                                 
     Investments at Fair Value as Determined
     By Quoted Market Price

     Managed Equity Funds                                                                 $80,204,814       $ (88,428,670)
     Managed International Equity Fund                                                      6,003,529          (3,262,394)
     Managed Fixed Income Fund                                                                284,375             600,215
     Ameren Common Stock Fund                                                              18,822,486          (2,662,341)
                                                                                     -----------------  ------------------

     Net change in fair value                                                             105,315,204         (93,753,190)

     Investments at Estimated Fair Value

     Managed Fixed Income Fund                                                                820,925                   -
                                                                                     -----------------  ------------------

     Net change in fair value                                                           $ 106,136,129       $ (93,753,190)
                                                                                     ================   ==================


4.   Nonparticipant-Directed Investments

     Information  about the net assets  and the  significant  components  of the
     changes in net assets relating to the  nonparticipant-directed  investments
     at and for the years ended December 31, 2003 and 2002, is as follows:


                                                                                             2003                2002

     Net assets
     Ameren Common Stock Fund                                                            $47,387,033         $40,209,751
     Employer contributions receivable                                                       116,042             182,310

     Changes in net assets
     Dividends                                                                             2,471,469           2,304,531
     Net appreciation in fair value of investments                                         4,392,749             703,956
     Employer contributions                                                                4,643,459           4,978,154
     Benefits paid to Participants                                                         2,859,756             971,143



5.   Transactions with Parties-in-Interest

     At December  31, 2003,  the Plan held Company  common stock with a cost and
     market value of $139,604,045 and $186,997,506,  respectively.  During 2003,
     the Plan purchased  shares at a cost of $24,658,853  and sold shares valued
     at $28,854,979.

     At December  31, 2002,  the Plan held Company  common stock with a cost and
     market value of $142,754,115 and $179,034,216,  respectively.  During 2002,
     the Plan purchased  shares at a cost of $25,044,373  and sold shares valued
     at $20,945,271.



                                       9




Ameren Corporation
Savings Investment Plan
Notes to Financial Statements
December 31, 2003 and 2002
--------------------------------------------------------------------------------

     The Plan  held  $4,537,265  and  $3,904,650  in  Northern  Trust  Company's
     Collective  Short-Term  Investment  Fund at  December  31,  2003 and  2002,
     respectively, which is managed by an affiliate of the Trustee.

     The Plan held  $83,122,275 in Northern  Trust  Company's  Collective  Trust
     Stable Asset Fund at December 31, 2003, which is managed by an affiliate of
     the Trustee.

     These  transactions  are  allowable  party-in-interest  transactions  under
     Section 408(b)(8) of the ERISA regulations.

6.   Reconciliation of Financial Statements to Form 5500

     The following is a reconciliation  of net assets available for benefits per
     the financial statements to the Form 5500 at December 31, 2003 and 2002:




                                                                                   2003                2002

                                                                                        
     Net assets available for benefits per the
      financial statements                                                      $735,918,742        $629,624,159
     Amounts allocated to withdrawing Participants                                  (502,676)           (213,431)
                                                                          -------------------  ------------------

     Net assets available for benefits per the Form 5500                        $735,416,066        $629,410,728
                                                                          ===================  ==================


     The following is a reconciliation  of benefits paid to Participants per the
     financial statements to the Form 5500 for the years ended December 31, 2003
     and 2002:


                                                                                   2003                2002

     Benefits paid to Participants per the
      financial statements                                                      $ 77,744,983        $ 37,311,364
     Add:  Amounts allocated to withdrawing
      Participants during the current year                                           502,676             213,431
     Less:  Amounts allocated to withdrawing
      Participants during the prior year                                            (213,431)           (428,138)
                                                                          -------------------  ------------------

     Benefits paid to Participants per the Form 5500                            $ 78,034,228        $ 37,096,657
                                                                          ===================  ==================



     Amounts allocated to withdrawing Participants are recorded on the Form 5500
     for benefit  claims that have been processed and approved for payment prior
     to December 31, but not yet paid as of that date.

7.   Federal Income Tax Status

     The Company  obtained its latest  determination  letter July 31,  2001,  in
     which the Internal  Revenue  Service stated that the Plan was in compliance
     with the  applicable  requirements  of the Code.  The Plan has been amended
     since receiving the  determination  letter.  However,  the Company believes
     that the Plan currently is designed and being  operated in compliance  with
     the applicable requirements of the Code and, therefore,  the Plan continues
     to qualify  under  Section  401(a) and the related  trust  continues  to be
     tax-exempt  as of December 31,  2003.  Therefore,  no provision  for income
     taxes has been included in the Plan's financial statements.

                                       10





Ameren Corporation
Savings Investment Plan
Schedule of Assets (Held at End of Year)
December 31, 2003                                                                                                         Schedule I
------------------------------------------------------------------------------------------------------------------------------------

 (a)                      (b)                                                (c)                                            (e)
                                                           Desciption of investment including maturity
    Identity of issue, borrower, lessor, or similar   date, rate of interest, collateral, par, or maturity                Current
                         party                                              value                                          value

                                                                                                               
*      Ameren Corporation                              Ameren Common Stock Fund                                       $186,997,506
       Lord Abbett                                     Lord Abbett Mid-Cap Value Fund                                  113,828,229
       Washington Mutual Investments                   Washington Mutual Investors Fund                                 90,778,245
*      Northern Trust Company                          Collective Trust Stable Asset Fund                               83,122,275
       Vanguard Group                                  Vanguard Asset Allocation Fund                                   68,985,586
       Barclays Global Investment Funds, Inc.          Barclays Global Investors Equity Index Fund                      48,503,305
       The American Funds                              World Fund of America                                            33,873,792
       The American Funds                              Europacific Growth Fund                                          28,383,017
       Vanguard Group                                  Vanguard Extended Market Index Fund                              24,257,077
*      T. Rowe Price Stable Asset Management, Inc.     T. Rowe Price Stable Value Common Trust Fund                     19,816,654
*  **  Participants                                    Participant Loans                                                17,728,136
       Pacific Investment Management Company           PIMCO Total Return Fund                                          13,713,848
*      Northern Trust Company                          Collective Short-term Investment Fund                             4,537,265
                                                                                                                      ------------

                                                                                                                      $734,524,935
                                                                                                                      ============


*       Investment represents allowable transaction with a party-in-interest.
**      Interest rates vary from 4.25 percent to 11.62 percent on loans maturing through 2016.

Note:   Information pertaining to column (d) was not available for nonparticipant-directed investments,
        and was omitted for participant-directed investments because it was not applicable.



                                       11






Ameren Corporation
Savings Investment Plan
Schedule H, Line 4j - Schedule of Reportable Transactions*
December 31, 2003                                                                                                        Schedule II
------------------------------------------------------------------------------------------------------------------------------------

                               (b)
                          Decription of                                              (f)                      (h)
    (a)              asset (include interest     (c)           (d)      (e)        Expense        (g)   Current value of      (i)
Identity of            rate and maturity in    Purchase      Selling   Lease    incurred with  Cost of      asset on       Net gain
party involved            case of a loan)       price         price    rental    transaction    asset   transaction date   or (loss)


Services of
Transactions
------------------
                                                                                                
Ameren Corporation   Ameren Common Stock Fund $ 24,658,853  $          -  $   -   $     -   $ 24,658,853 $  24,658,853  $         -
                                                         -    28,854,979      -         -     22,768,411    28,854,979    6,086,568

Northern Trust       Colletive Short-term      114,256,436             -      -         -    114,256,436   114,256,436            -
                       Investment Fund                   -   113,623,816      -         -    113,623,816   113,623,816            -

American Funds       Europacific Growth Fund    32,139,390             -      -         -     32,139,390    32,139,390            -
                                                         -     2,815,495      -         -      2,617,422     2,815,495      198,073

Lord Abbett          Lord Abbett Mid-Cap       104,887,107             -      -         -    104,887,107   104,887,107            -
                       Value Fund                        -     7,230,529      -         -      5,693,171     7,230,529    1,537,358

Morgan Stanley       MSIF Trust Fund             5,860,426             -      -         -      5,860,426     5,860,426            -
                                                         -   103,248,383      -         -    131,815,129   103,248,383  (28,566,747)

T. Rowe Price        T. Rowe Price Stable      102,377,958             -      -         -    102,377,958   102,377,958            -
                       Value Common Trust Fund           -   191,852,148      -         -    191,852,148   191,852,148            -

Vanguard             Vanguard U.S. Growth Fund  4,484,398              -      -         -      4,484,398     4,484,398            -
                                                        -     31,113,888      -         -     62,552,248    31,113,888  (31,438,361)

Individual
Transactions
------------------

T. Rowe Price               T. Rowe Price Stable        -     78,198,322      -         -     78,198,322             -            -
                              Value Common Trust Fund

T. Rowe Price               T. Rowe Price Stable 78,198,322            -      -         -     78,198,322             -            -
                              Value Common Trust Fund

Lord Abbett                 Lord Abbett Mid-Cap  95,751,282            -      -         -     95,751,282             -            -
                              Value Fund

Morgan Stanley              MSIF Trust Fund               -   96,479,464      -         -    121,503,144             -  (25,023,679)





*Note:  Transactions or a series of transactions in excess of 5 percent of the current value of the Plan's assets as of the
 beginning of the plan year as defined in Section 29 CFR 2520.103-6 of the Department of Labor Rules and Regulations for Reporting
 and Disclosure under ERISA.



                                       12



                                   SIGNATURES

          The Plan.  Pursuant to the requirements of the Securities Exchange Act

of 1934,  the trustees (or other  persons who  administer  the employee  benefit

plan)  have duly  caused  this  annual  report to be signed on its behalf by the

undersigned hereunto duly authorized.


                                        AMEREN CORPORATION
                                        SAVINGS INVESTMENT PLAN



                                        AMEREN SERVICES COMPANY
                                            (Administrator)




                                        By   /s/ Donna K. Martin
                                          --------------------------------------
                                                 Donna K. Martin
                                                 Vice President

June 28, 2004




                                  EXHIBIT INDEX




Exhibit No.                       Description
-----------           -----------------------------------------

    23                Consent of Independent Auditors