Exhibit 31.1  Page 1
of 2

RULE
13a-14(a)/15d-14(a) CERTIFICATION 
OF PRINCIPAL EXECUTIVE OFFICER OF AMEREN CORPORATION  (required by Section 302 of the Sarbanes-Oxley Act of 2002)

I,
Gary L. Rainwater, certify that:

1.       
          I have reviewed this report on Form 10-K for the fiscal year ended
          December 31, 2003 of Ameren Corporation;

2.       
          Based on my knowledge, this report does not contain any untrue statement of a
          material fact or omit to state a material fact necessary to make the statements
          made, in light of the circumstances under which such statements were made, not
          misleading with respect to the period covered by this report;

3.       
          Based on my knowledge, the financial statements, and other financial information
          included in this report, fairly present in all material respects the financial
          condition, results of operations and cash flows of the registrant as of, and
          for, the periods presented in this report;

4.       
          The registrants other certifying officer and I are responsible for
          establishing and maintaining disclosure controls and procedures (as defined in
          Exchange Act Rules 13a-15(e) and 15d-15(e)) for the registrant and have:

|a) |  | Designed such disclosure controls and procedures, or caused such disclosure
                    controls and procedures to be designed under our supervision, to ensure that
                    material information relating to the registrant, including its consolidated
                    subsidiaries, is made known to us by others within those entities, particularly
                    during the period in which this report is being prepared;
|-- |  | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

|b) |  | Evaluated the effectiveness of the registrants disclosure controls and
                    procedures and presented in this report our conclusions about the effectiveness
                    of the disclosure controls and procedures, as of the end of the period covered
                    by this report based on such evaluation; and
|-- |  | -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

|c) |  | Disclosed in this report any change in the registrants internal control
                    over financial reporting that occurred during the registrants most recent
                    fiscal quarter (the registrants fourth fiscal quarter in the case of an
                    annual report) that has materially affected, or is reasonably likely to
                    materially affect, the registrants internal control over financial
                    reporting; and
|-- |  | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

5.       
          The registrants other certifying officer and I have disclosed, based on
          our most recent evaluation of internal control over financial reporting, to the
          registrants auditors and the audit committee of registrants board of
          directors (or persons performing the equivalent functions):

|a) |  | All significant deficiencies and material weaknesses in the design or operation
                    of internal control which are reasonably likely to adversely affect the
                    registrants ability to record, process, summarize and report financial
                    information; and
|-- |  | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

---

Exhibit 31.1 Page 2
of 2

|b) |  | Any fraud, whether or not material, that involves management or other employees
                    who have a significant role in the registrants internal control over
                    financial reporting.
|-- |  | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Date: March 9, 2004

| |  | /s/ Gary L. Rainwater
Gary L. RainwaterChairman, President and  Chief Executive Officer(Principal Executive Officer)
| |  | ----------------------------------------------------------------------------------------------------------------------------------