SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549






                                   FORM U5S/A

                                 AMENDMENT NO. 1



                                  ANNUAL REPORT

                      For the Year Ended December 31, 2003






       Filed pursuant to the Public Utility Holding Company Act of 1935 by









                               AMEREN CORPORATION
                 1901 Chouteau Avenue, St. Louis, Missouri 63103





     This Amendment No. 1 to Ameren Corporation  (Ameren) Form U5S Annual Report
     for the year ended  December 31, 2003,  is being filed to update Item 10 to
     include the financial  statements of  AmerenEnergy  Medina Valley Cogen LLC
     that were not included in Ameren's Form U5S filed on April 29, 2003.


                                 Table of Contents

ITEM NUMBER                DESCRIPTION                             PAGE NUMBER

   10                  Financial Statements and Exhibits               51







ITEM 10.  FINANCIAL STATEMENTS AND EXHIBITS

H.   FINANCIAL STATEMENTS FOR EACH EWG OF THE REGISTERED
     HOLDING COMPANY

      H.5  Unaudited  Financial  Statements  for  AmerenEnergy  Medina Valley
           Cogen LLC (filed confidentially on Form SE)








SIGNATURE


     Ameren  Corporation,  a registered  holding  company,  has duly caused this
     Amendment No. 1 to the Annual  Report for the year ended  December 31, 2003
     to be signed on its behalf by the  undersigned  thereunto duly  authorized,
     pursuant to the  requirements  of the Public Utility Holding Company Act of
     1935.



                                             AMEREN CORPORATION


                                             By: /s/ Martin J. Lyons, Jr.
                                                ---------------------------
                                                Martin J. Lyons, Jr.
                                                Vice President and Controller

July 1, 2004