AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON MARCH 12, 2002.
                                                      REGISTRATION NO. 333-81774
================================================================================


                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

                              ---------------------

                                 POST-EFFECTIVE
                                 AMENDMENT NO. 1
                                       TO
                                    FORM S-3

                             REGISTRATION STATEMENT
                                      UNDER
                           THE SECURITIES ACT OF 1933

                              ---------------------

                               AMEREN CORPORATION
             (Exact name of registrant as specified in its charter)

            MISSOURI                                     43-1723446
(State or other jurisdiction of             (I.R.S. Employer Identification No.)
 incorporation or organization)

                              1901 CHOUTEAU AVENUE,
                            ST. LOUIS, MISSOURI 63103
                                 (314) 621-3222
          (Address, including zip code, and telephone number, including
             area code, of registrant's principal executive offices)

                                WARNER L. BAXTER
                         SENIOR VICE PRESIDENT, FINANCE

                               STEVEN R. SULLIVAN
                  VICE PRESIDENT, GENERAL COUNSEL AND SECRETARY
                              1901 CHOUTEAU AVENUE
                            ST. LOUIS, MISSOURI 63103
                                 (314) 621-3222
            (Name, address, including zip code, and telephone number,
                   including area code, of agents for service)


================================================================================




               PART II. INFORMATION NOT REQUIRED IN THE PROSPECTUS

ITEM 16.  Exhibits

EXHIBIT NO.                            DESCRIPTION
-----------  -------------------------------------------------------------------

***1.1    Form of Underwriting Agreement relating to the debt securities.

  +1.2    Form of Underwriting Agreement relating to the trust preferred
          securities.

***1.3    Form of Underwriting Agreement relating to the common stock.

  *1.4    Underwriting Agreement, dated February 26, 2002, among the Company and
          several underwriters named therein relating to the equity security
          units.

 **4.1    Restated Articles of Incorporation of the Company (File No. 33-64165,
          Annex F).

 **4.2    Certificate of Amendment to the Restated Articles of Incorporation
          filed with the Secretary of State of the State of Missouri on December
          14, 1998 (1998 Form 10-K, Exhibit 3(i)).

 **4.3    By-laws of the Company as amended to December 31, 1997 (1997 Form
          10-K, Exhibit 3(ii)).

 **4.4    Agreement, dated as of October 9, 1998, between the Company and
          EquiServe Trust Company, N.A. (as successor to First Chicago Trust
          Company of New York), as Rights Agent, which includes the form of
          Certificate of Designation of the Preferred Shares as Exhibit A, the
          form of Rights Certificate as Exhibit B and the Summary of Rights as
          Exhibit C (October 14, 1998 Form 8-K, Exhibit 4).

***4.5    Indenture of the Company with The Bank of New York, as trustee,
          relating to the senior debt securities dated as of December 1, 2001.

***4.6    Company order relating to $150,000,000 Floating Rate Notes due
          December 12, 2003 issued under the senior indenture (including the
          forms of notes as exhibits).

***4.7    Company order relating to $100,000,000 5.70% Notes due February 1,
          2007 issued under the senior indenture (including the forms of notes
          as exhibits).

  *4.8    Company order relating to $345,000,000 Notes due May 15, 2007 issued
          under the senior indenture (including the forms of notes as exhibits).

***4.9    Form of Indenture of the Company relating to subordinated debt
          securities.

  +4.10   Form of supplemental indenture or other instrument establishing the
          issuance of one or more series of subordinated debt securities
          (including the form of subordinated debt security).

***4.11   Form of Guarantee Agreement of the Company.

***4.12   Form of Certificate of Trust.

***4.13   Form of Trust Agreement.

***4.14   Form of Amended and Restated Trust Agreement (including the form of
          trust preferred security as an exhibit).


                                      II-1



EXHIBIT NO.                            DESCRIPTION
-----------  -------------------------------------------------------------------

  *4.15   Purchase Contract Agreement dated as of March 1, 2002, between the
          Company and The Bank of New York, as purchase contract agent.

  *4.16   Pledge Agreement dated as of March 1, 2002, among the Company, The
          Bank of New York, as purchase contract agent and BNY Trust Company of
          Missouri, as custodial agent, collateral agent and securities
          intermediary.

  +4.17   Form of Warrant Agreement (including form of warrant).

  *4.18   Remarketing Agreement dated as of March 4, 2002, among the Company,
          The Bank of New York, as purchase contract agent and Goldman, Sachs &
          Co., as remarketing agent.

***5.1    Opinion of Steven R. Sullivan, Esq., Vice President, General Counsel
          and Secretary of the Company, regarding the legality of the
          securities.

***5.2    Opinion of Pillsbury Winthrop LLP regarding the legality of the
          securities.

***12     Statement re computation of ratios of earnings to fixed charges.

***23.1   Consent of Steven R. Sullivan, Esq., Vice President, General Counsel
          and Secretary of the Company (included in Exhibit 5.1).

***23.2   Consent of Pillsbury Winthrop LLP (included in Exhibit 5.2).

***23.3   Consent of independent accountants.

***24     Powers of attorney.

***25.1   Form T-1 statement of eligibility of the trustee for the senior debt
          securities.

 ++25.2   Form T-1 statement of eligibility of the trustee for the subordinated
          debt securities.

 ++25.3   Form T-1 statement of eligibility of the trustee for the guarantees
          for the benefit of the holders of the trust preferred securities.

 ++25.4   Form T-1 statement of eligibility of the trustee for the trust
          preferred securities.

***25.5   Form T-1 statement of eligibility of the purchase contract agent for
          the stock purchase contracts.

***99.1   Form of prospectus supplement with respect to equity security units.

***99.2   Form of prospectus supplement with respect to common stock.

NOTE: REPORTS OF THE COMPANY ON FORMS 8-K, 10-Q AND 10-K ARE ON FILE WITH THE
SEC UNDER FILE NUMBER 1-14756.

----------

*    Filed herewith.
**   Incorporated by reference herein as indicated.
***  Previously Filed.
+    To be filed by amendment or pursuant to a report to be filed pursuant to
     Section 13 or 15(d) of the Securities Exchange Act of 1934.
++   To be filed by amendment or pursuant to Trust Indenture Act Section
     305(b)(2).


                                      II-2



                                   SIGNATURES

     Pursuant to the requirements of the Securities Act of 1933, the Company
certifies that it has reasonable grounds to believe that it meets all of the
requirements for filing on Form S-3 and has duly caused this registration
statement to be signed on its behalf by the undersigned, thereunto duly
authorized, in the City of St. Louis, and State of Missouri, on the 12th day of
March 2002.

                                        AMEREN CORPORATION (Registrant)

                                        By: /s/ Charles W. Mueller
                                        ----------------------------------------
                                            CHARLES W. MUELLER
                                            Chairman and Chief Executive Officer

     Pursuant to the requirements of the Securities Act of 1933, this
registration statement has been signed by the following persons in the
capacities and on the date indicated.

Name                          Title                              Date
----                          -----                              ----

/s/ Charles W. Mueller        Chairman and                       March 12, 2002
---------------------------   Chief Executive Officer
CHARLES W. MUELLER            (Principal Executive Officer)


/s/ Gary L. Rainwater         President and                      March 12, 2002
---------------------------   Chief Operating Officer
GARY L. RAINWATER


/s/ Warner L. Baxter          Senior Vice President, Finance     March 12, 2002
---------------------------   (Principal Financial Officer)
WARNER L. BAXTER


/s/ Martin J. Lyons, Jr.      Controller                         March 12, 2002
---------------------------   (Principal Accounting Officer)
MARTIN J. LYONS, JR.


             *                Director                           March 12, 2002
---------------------------
WILLIAM E. CORNELIUS


             *                Director                           March 12, 2002
---------------------------
CLIFFORD L. GREENWALT


             *                Director                           March 12, 2002
---------------------------
THOMAS A. HAYS


                                      II-3



             *                Director                           March 12, 2002
---------------------------
THOMAS H. JACOBSEN


             *                Director                           March 12, 2002
---------------------------
RICHARD A. LIDDY


             *                Director                           March 12, 2002
---------------------------
GORDON R. LOHMAN


             *                Director                           March 12, 2002
---------------------------
RICHARD A. LUMPKIN


             *                Director                           March 12, 2002
---------------------------
JOHN PETERS MacCARTHY


             *                Director                           March 12, 2002
---------------------------
HANNE M. MERRIMAN


             *                Director                           March 12, 2002
---------------------------
PAUL L. MILLER, JR.


             *                Director                           March 12, 2002
---------------------------
HARVEY SALIGMAN


             *                Director                           March 12, 2002
---------------------------
JANET McAFEE WEAKLEY


             *                Director                           March 12, 2002
---------------------------
JAMES W. WOGSLAND


*  By: /s/ Steven R. Sullivan
       -------------------------
       STEVEN R. SULLIVAN
       Attorney-in-Fact


                                      II-4



                                  EXHIBIT INDEX

EXHIBIT NO.                            DESCRIPTION
-----------  -------------------------------------------------------------------

***1.1    Form of Underwriting Agreement relating to the debt securities.

  +1.2    Form of Underwriting Agreement relating to the trust preferred
          securities.

***1.3    Form of Underwriting Agreement relating to the common stock.

  *1.4    Underwriting Agreement, dated February 26, 2002, among the Company and
          several underwriters named therein relating to the equity security
          units.

 **4.1    Restated Articles of Incorporation of the Company (File No. 33-64165,
          Annex F).

 **4.2    Certificate of Amendment to the Restated Articles of Incorporation
          filed with the Secretary of State of the State of Missouri on December
          14, 1998 (1998 Form 10-K, Exhibit 3(i)).

 **4.3    By-laws of the Company as amended to December 31, 1997 (1997 Form
          10-K, Exhibit 3(ii)).

 **4.4    Agreement, dated as of October 9, 1998, between the Company and
          EquiServe Trust Company, N.A. (as successor to First Chicago Trust
          Company of New York), as Rights Agent, which includes the form of
          Certificate of Designation of the Preferred Shares as Exhibit A, the
          form of Rights Certificate as Exhibit B and the Summary of Rights as
          Exhibit C (October 14, 1998 Form 8-K, Exhibit 4).

***4.5    Indenture of the Company with The Bank of New York, as trustee,
          relating to the senior debt securities dated as of December 1, 2001.

***4.6    Company order relating to $150,000,000 Floating Rate Notes due
          December 12, 2003 issued under the senior indenture (including the
          forms of notes as exhibits).

***4.7    Company order relating to $100,000,000 5.70% Notes due February 1,
          2007 issued under the senior indenture (including the forms of notes
          as exhibits).

  *4.8    Company order relating to $345,000,000 Notes due May 15, 2007 issued
          under the senior indenture (including the forms of notes as exhibits).

***4.9    Form of Indenture of the Company relating to subordinated debt
          securities.

  +4.10   Form of supplemental indenture or other instrument establishing the
          issuance of one or more series of subordinated debt securities
          (including the form of subordinated debt security).

***4.11   Form of Guarantee Agreement of the Company.

***4.12   Form of Certificate of Trust.

***4.13   Form of Trust Agreement.

***4.14   Form of Amended and Restated Trust Agreement (including the form of
          trust preferred security as an exhibit).

  *4.15   Purchase Contract Agreement dated as of March 1, 2002, between the
          Company and The Bank of New York, as purchase contract agent.




  *4.16   Pledge Agreement dated as of March 1, 2002, among the Company, The
          Bank of New York, as purchase contract agent and BNY Trust Company of
          Missouri, as custodial agent, collateral agent and securities
          intermediary.

  +4.17   Form of Warrant Agreement (including form of warrant).

  *4.18   Remarketing Agreement dated as of March 4, 2002, among the Company,
          The Bank of New York, as purchase contract agent and Goldman, Sachs &
          Co., as remarketing agent.

***5.1    Opinion of Steven R. Sullivan, Esq., Vice President, General Counsel
          and Secretary of the Company, regarding the legality of the
          securities.

***5.2    Opinion of Pillsbury Winthrop LLP regarding the legality of the
          securities.

***12     Statement re computation of ratios of earnings to fixed charges.

***23.1   Consent of Steven R. Sullivan, Esq., Vice President, General Counsel
          and Secretary of the Company (included in Exhibit 5.1).

***23.2   Consent of Pillsbury Winthrop LLP (included in Exhibit 5.2).

***23.3   Consent of independent accountants.

***24     Powers of attorney.

***25.1   Form T-1 statement of eligibility of the trustee for the senior debt
          securities.

 ++25.2   Form T-1 statement of eligibility of the trustee for the subordinated
          debt securities.

 ++25.3   Form T-1 statement of eligibility of the trustee for the guarantees
          for the benefit of the holders of the trust preferred securities.

 ++25.4   Form T-1 statement of eligibility of the trustee for the trust
          preferred securities.

***25.5   Form T-1 statement of eligibility of the purchase contract agent for
          the stock purchase contracts.

***99.1   Form of prospectus supplement with respect to equity security units.

***99.2   Form of prospectus supplement with respect to common stock.

NOTE: REPORTS OF THE COMPANY ON FORMS 8-K, 10-Q AND 10-K ARE ON FILE WITH THE
SEC UNDER FILE NUMBER 1-14756.

----------

*    Filed herewith.
**   Incorporated by reference herein as indicated.
***  Previously Filed.
+    To be filed by amendment or pursuant to a report to be filed pursuant to
     Section 13 or 15(d) of the Securities Exchange Act of 1934.
++   To be filed by amendment or pursuant to Trust Indenture Act Section
     305(b)(2).


                                       2