<Page>

                                                                     EXHIBIT 4.6

                                  COMPANY ORDER

                                                         December 12, 2001


The Bank of New York
c/o BNY Trust Company of Missouri
911 Washington Street
St. Louis, Missouri 63101

Ladies and Gentlemen:

      Application is hereby made to The Bank of New York, a New York banking
corporation, as trustee (the "Trustee"), under the Indenture dated as of
December 1, 2001 (the "Indenture"), between Ameren Corporation, a Missouri
corporation (the "Company"), and the Trustee for the authentication and delivery
of $150,000,000 aggregate principal amount of the Company's Floating Rate Notes
due December 12, 2003 (the "Notes"), pursuant to the provisions of Article II of
the Indenture. Additional Notes without limitation as to amount, and without the
consent of the holders of the then Outstanding Notes, may also be authenticated
and delivered in the manner provided in Section 2.05 of the Indenture. All
capitalized terms not defined herein that are defined in the Indenture shall
have the same meaning as used in the Indenture.

      The Notes will be initially issued pursuant to Section 4(2) of the
Securities Act of 1933, as amended (the "Securities Act"), in the form of Global
Notes registered in the name of Cede & Co. (as nominee for The Depository Trust
Company ("DTC"), New York, New York, which will act as the Depositary for the
Global Notes). Pursuant to Section 2.05(c) of the Indenture, the Notes will have
the terms set forth in the form of Global Note attached hereto as Exhibit A and
in the form of definitive Note attached hereto as Exhibit B (which terms are
incorporated by reference in this Company Order). The Global Notes shall bear
the depository legend in substantially the form set forth in Exhibit A attached
hereto. The Notes shall be subject to restrictions on transfer, and each Note,
whether in global form or in definitive form, shall bear the non-registration
legend set forth in substantially the form in the applicable form of Note,
unless otherwise agreed by the Company, such agreement to be confirmed in
writing to the Trustee. Nothing in the Indenture or the Notes shall be construed
to require the Company to register any of the Notes under the Securities Act,
unless otherwise expressly agreed by the Company, confirmed in writing to the
Trustee, or to make any transfer of the Notes or the beneficial interests
therein in violation of applicable law. The Notes will be issued only in
denominations of $100,000 and in integral multiples of $10,000 in excess
thereof.

      It is contemplated that beneficial interests in the Notes owned by
qualified institutional buyers (as defined in Rule 144A under the Securities
Act) ("QIBs") or sold to QIBs in reliance upon Rule 144A under the Securities
Act will be represented by one or more separate Global Notes (each, a "Rule 144A
Global Certificate") registered in the name of Cede & Co., as registered owner
and as nominee for DTC; beneficial interests in the Notes sold to purchasers
pursuant to Regulation S under the Securities Act will be evidenced by one or
more separate Global Notes (each, a "Regulation S Global Certificate") and will
be registered in the name of

<Page>

Cede & Co., as registered owner and as nominee for DTC for the accounts of The
Euroclear System or Clearstream Banking, Luxembourg, societe anonyme; prior to
the 40th day after the date of initial issuance of the Notes, beneficial
interests in a Regulation S Global Certificate may be held only through The
Euroclear System or Clearstream Banking, Luxembourg, societe anonyme (unless a
resale or transfer is effected pursuant to Rule 144A under the Securities Act);
Notes acquired by Institutional Accredited Investors (as defined in Rule
501(a)(1), (2), (3) or (7) under the Securities Act) ("IAIs"), who are not QIBs
and who are not purchasers pursuant to Regulation S under the Securities Act,
will be in definitive form. The Trustee and the Company will have no
responsibility or liability for any aspect of transfers of beneficial interests
in the Notes (which transfers will be conducted pursuant to the customary
procedures of DTC), any records of DTC of beneficial interests or any
transactions between DTC and its participants or between any such participants
and any other beneficial owners or for monitoring, supervising or reviewing of
any thereof.

      In connection with any transfer of Notes, the Trustee and the Company
shall be under no duty to inquire into, may conclusively presume the correctness
of, and shall be fully protected in relying upon the certificates and other
information (set forth in the form of definitive Note attached hereto as Exhibit
B, for use in connection with the transfer of the Notes in definitive form, or
set forth in Exhibit A-1 attached hereto, for use in connection with the
transfer of beneficial interests between a Rule 144A Global Certificate and a
Regulation S Global Certificate or to a Note in definitive form, or otherwise)
received from the Holders and any transferees of any Notes regarding the
validity, legality and due authorization of any such transfer, the eligibility
of the transferee to receive such Note and any other facts and circumstances
related to such transfer. Transfers of beneficial interests between a Rule 144A
Global Certificate and a Regulation S Global Certificate, and other transfers
relating to beneficial interests in the Notes in global form, shall be reflected
by endorsements of the Trustee, as custodian for DTC, on the schedule attached
to such certificate.

      In connection with this Company Order, there are delivered to you herewith
the following:

      1.    Certified copies of the resolutions adopted by the Board of
            Directors of the Company and by the Executive Committee of the Board
            of Directors of the Company authorizing this Company Order and the
            issuance and sale of the Notes by the Company pursuant to Section
            2.05(c)(1) of the Indenture;

      2.    Opinions of Counsel addressed to you or in which it is stated that
            you may rely pursuant to Section 2.05(c)(2) of the Indenture;

      3.    Officers' Certificate pursuant to Section 2.05(c)(3) of the
            Indenture; and

      4.    Two Global Notes representing the Notes executed on behalf of the
            Company in accordance with the terms of Section 2.05(a) of the
            Indenture.

      You are hereby instructed to authenticate the Global Notes representing
the Notes and hold them as DTC's custodian. The Global Notes representing the
Notes are to be held for delivery through the facilities of DTC to the initial
purchasers thereof against payment therefor at


                                       2
<Page>

the closing in respect of the sale thereof, such closing to be held at 10:00
a.m., New York time, December 12, 2001, at the offices of Pillsbury Winthrop
LLP, One Battery Park Plaza, New York, NY 10004.

      Please acknowledge receipt of the Global Notes representing the Notes, the
instructions referred to above and the supporting documentation pursuant to the
Indenture referred to above.

                                     Very truly yours,


                                     Ameren Corporation

                                     By:  /s/ Jerre E. Birdsong
                                        ---------------------------
                                        Name:  Jerre E. Birdsong
                                        Title: Vice President and Treasurer

      Receipt from the Company of the Global Notes representing the Notes,
certain instructions related thereto and the supporting documentation pursuant
to the Indenture in connection with the authentication and delivery of the Notes
is hereby acknowledged.

                                     The Bank of New York,
                                     as Trustee


                                     By:  /s/ Robert A. Massimillo
                                        ------------------------------
                                        Name:  Robert A. Massimillo
                                        Title: Vice President


                                       3
<Page>

                                    EXHIBIT A

                               FORM OF GLOBAL NOTE

      THIS NOTE IS A GLOBAL NOTE REGISTERED IN THE NAME OF THE DEPOSITARY
(REFERRED TO HEREIN) OR A NOMINEE THEREOF AND, UNLESS AND UNTIL IT IS EXCHANGED
IN WHOLE FOR THE INDIVIDUAL NOTES REPRESENTED HEREBY AS PROVIDED IN THE
INDENTURE REFERRED TO BELOW, THIS NOTE MAY NOT BE TRANSFERRED EXCEPT AS A WHOLE
BY THE DEPOSITARY TO A NOMINEE OF THE DEPOSITARY OR BY A NOMINEE OF THE
DEPOSITARY TO THE DEPOSITARY OR ANOTHER NOMINEE OF THE DEPOSITARY OR BY THE
DEPOSITARY OR ANY SUCH NOMINEE TO A SUCCESSOR DEPOSITARY OR A NOMINEE OF SUCH
SUCCESSOR DEPOSITARY. UNLESS THIS NOTE IS PRESENTED BY AN AUTHORIZED
REPRESENTATIVE OF THE DEPOSITORY TRUST COMPANY (55 WATER STREET, NEW YORK, NEW
YORK), TO THE TRUSTEE FOR REGISTRATION OF TRANSFER, EXCHANGE OR PAYMENT, AND ANY
CERTIFICATE ISSUED IS REGISTERED IN THE NAME OF CEDE & CO. OR SUCH OTHER NAME AS
REQUESTED BY AN AUTHORIZED REPRESENTATIVE OF THE DEPOSITORY TRUST COMPANY AND
ANY PAYMENT IS MADE TO CEDE & CO., ANY TRANSFER, PLEDGE OR OTHER USE HEREOF FOR
VALUE OR OTHERWISE BY OR TO ANY PERSON IS WRONGFUL SINCE THE REGISTERED OWNER
HEREOF, CEDE & CO., HAS AN INTEREST HEREIN.

      THIS SECURITY HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS
AMENDED (THE "SECURITIES ACT"). THE HOLDER HEREOF, BY PURCHASING THIS SECURITY,
AGREES FOR THE BENEFIT OF THE COMPANY THAT THIS SECURITY MAY NOT BE RESOLD,
PLEDGED OR OTHERWISE TRANSFERRED PRIOR TO THE DATE WHICH IS TWO YEARS AFTER THE
LATER OF THE ORIGINAL ISSUANCE DATE THEREOF AND THE LAST DATE ON WHICH THE
COMPANY OR ANY AFFILIATE THEREOF WAS THE OWNER OF THIS SECURITY OR THE
EXPIRATION OF SUCH SHORTER PERIOD AS MAY BE PRESCRIBED BY RULE 144(k), OR ANY
SUCCESSOR PROVISION THEREOF, UNDER THE SECURITIES ACT (THE "RESALE RESTRICTION
TERMINATION DATE") OTHER THAN (1) TO THE COMPANY, (2) IN A TRANSACTION ENTITLED
TO AN EXEMPTION FROM REGISTRATION PROVIDED BY RULE 144 UNDER THE SECURITIES ACT,
(3) SO LONG AS THIS SECURITY IS ELIGIBLE FOR RESALE PURSUANT TO RULE 144A UNDER
THE SECURITIES ACT ("RULE 144A"), TO A PERSON WHOM THE SELLER REASONABLY
BELIEVES IS A QUALIFIED INSTITUTIONAL BUYER WITHIN THE MEANING OF RULE 144A
PURCHASING FOR ITS OWN ACCOUNT OR FOR THE ACCOUNT OF A QUALIFIED INSTITUTIONAL
BUYER TO WHOM NOTICE IS GIVEN THAT THE RESALE, PLEDGE OR OTHER TRANSFER IS BEING
MADE IN RELIANCE ON RULE 144A, (4) IN AN OFFSHORE TRANSACTION IN ACCORDANCE WITH
RULE 904 OF REGULATION S UNDER THE SECURITIES ACT, (5) TO AN INSTITUTION THAT IS
AN "ACCREDITED INVESTOR" AS DEFINED IN RULE 501(a)(1), (2), (3) OR (7) UNDER THE
SECURITIES ACT THAT IS ACQUIRING THIS SECURITY FOR INVESTMENT PURPOSES AND NOT
FOR DISTRIBUTION, AND A CERTIFICATE IN THE FORM ATTACHED TO THIS SECURITY IS
DELIVERED BY THE TRANSFEREE TO


                                      A-1
<Page>

THE COMPANY AND THE TRUSTEE OR (6) IN ACCORDANCE WITH ANOTHER APPLICABLE
EXEMPTION FROM, OR IN A TRANSACTION NOT SUBJECT TO, THE REGISTRATION
REQUIREMENTS OF THE SECURITIES ACT OR PURSUANT TO AN EFFECTIVE REGISTRATION
STATEMENT UNDER THE SECURITIES ACT, IN EACH CASE IN ACCORDANCE WITH ANY
APPLICABLE SECURITIES LAWS OF ANY STATE OF THE UNITED STATES. THE FOREGOING
RESTRICTIONS ON RESALE WILL NOT APPLY SUBSEQUENT TO THE RESALE RESTRICTION
TERMINATION DATE.

                               AMEREN CORPORATION
                    FLOATING RATE NOTE DUE DECEMBER 12, 2003

CUSIP/ISIN:                                  NUMBER:

ORIGINAL ISSUE DATE: December 12, 2001       PRINCIPAL AMOUNT: Listed on
                                                               Schedule I hereto

INTEREST RATE: Floating, as set forth below  MATURITY DATE: December 12, 2003

      AMEREN CORPORATION, a corporation of the State of Missouri (the
"COMPANY"), for value received hereby promises to pay to CEDE & CO. or
registered assigns, the principal amount specified above on the Maturity Date
set forth above, and to pay interest thereon from and including the Original
Issue Date specified above or from and including the most recent Interest
Payment Date to which interest has been paid or duly provided for, quarterly in
arrears on the March 12, June 12, September 12 and December 12 in each year,
commencing March 12, 2002, and on the Maturity Date, at the per annum Interest
Rate determined by the Calculation Agent on each Interest Determination Date, as
such terms are defined herein, until the principal hereof is paid or made
available for payment. No interest shall accrue on the Maturity Date, so long as
the principal amount of this Note is paid on the Maturity Date. The interest so
payable and punctually paid or duly provided for on any such Interest Payment
Date (except for interest payable on the Maturity Date set forth above or, if
applicable, upon redemption or acceleration) will, as provided in the Indenture
(as defined below), be paid to the Person in whose name this Note is registered
at the close of business on the Regular Record Date for such interest, which
shall be the fifteenth calendar day next preceding such Interest Payment Date;
provided, that the first Interest Payment Date for any part of this Note, the
Original Issue Date of which is after a Regular Record Date but prior to the
applicable Interest Payment Date, shall be the Interest Payment Date following
the next succeeding Regular Record Date; and provided, that interest payable on
the Maturity Date set forth above or, if applicable, upon redemption or
acceleration, shall be payable to the Person to whom principal shall be payable.
Except as otherwise provided in the Indenture, any such interest not so
punctually paid or duly provided for will forthwith cease to be payable to the
Holder on such Regular Record Date and shall be paid to the Person in whose name
this Note is registered at the close of business on a Special Record Date for
the payment of such defaulted interest to be fixed by the Trustee, notice
whereof shall be given to Noteholders not more than fifteen days or fewer than
ten days prior to such Special Record Date. Payment of the principal of and
interest and premium on this shall be payable pursuant to Section 2.12(a) of the
Indenture.


                                      A-2
<Page>

      This Note is a Global Note in respect of a duly authorized issue of
Floating Rate Notes due December 12, 2003 (the "NOTES OF THIS SERIES", which
term includes any Global Notes representing such Notes) of the Company issued
and to be issued under an Indenture dated as of December 1, 2001 between the
Company and The Bank of New York, as trustee (herein called the "TRUSTEE", which
term includes any successor Trustee under the Indenture) and indentures
supplemental thereto (collectively, the "INDENTURE"). Under the Indenture, one
or more series of notes may be issued and, as used herein, the term "Notes"
refers to the Notes of this Series. Reference is hereby made to the Indenture
for a more complete statement of the respective rights, limitations of rights,
duties and immunities thereunder of the Company, the Trustee and the Noteholders
and of the terms upon which the Notes are and are to be authenticated and
delivered. This Note has been issued in respect of the series designated on the
first page hereof, issued in the initial aggregate principal amount of
$150,000,000.

      Each Note of this Series shall be dated and issued as of the date of its
authentication by the Trustee and shall bear an Original Issue Date. Each Note
of this Series issued upon transfer, exchange or substitution of such Note shall
bear the Original Issue Date of such transferred, exchanged or substituted Note,
as the case may be.

      This Note will bear interest for each Interest Period at a per annum rate
("Interest Rate") determined by The Bank of New York, or its successor appointed
by the Company acting as calculation agent (the "Calculation Agent"), subject to
the maximum interest rate permitted by New York or other applicable state law,
as such law may be modified by United States law of general application. The
Interest Rate for each Interest Period will be equal to three month LIBOR on the
Interest Determination Date for such Interest Period plus 0.95%. Promptly upon
such determination, the Calculation Agent will notify the Trustee for the Notes,
if the Trustee is not then serving as the Calculation Agent, of the Interest
Rate for the new Interest Period. The Interest Rate determined by the
Calculation Agent, absent willful default, bad faith or manifest error, shall be
binding and conclusive upon the beneficial owners and holders of this Note, the
Company and the Trustee.

      Interest on this Note will accrue from and including the Original Issue
Date specified above to, but excluding, March 12, 2002, and thereafter, from and
including each Interest Payment Date to, but excluding, the next succeeding
Interest Payment Date or the Maturity Date, as the case may be.

      If the following circumstances exist on any Interest Determination Date,
the Calculation Agent shall determine LIBOR for the Notes as follows:

      (1) In the event no Reported Rate appears on Telerate Page 3750 as of
approximately 11:00 a.m. London time on an Interest Determination Date, the
Calculation Agent shall request the principal London offices of each of four
major banks in the London interbank market selected by the Calculation Agent
(after consultation with the Company) to provide a quotation of the rate at
which Three Month Deposits are offered by it to prime banks in the London
interbank market, as of approximately 11:00 a.m. London time on such Interest
Determination Date, that is representative of single transactions at such time
(the "Representative Amounts"). If at least two rate quotations are provided,
LIBOR will be the arithmetic mean of the rate quotations obtained by the
Calculation Agent.


                                      A-3
<Page>

      (2) In the event no Reported Rate appears on Telerate Page 3750 the
Calculation Agent obtains fewer than two rate quotations, LIBOR will be the
arithmetic mean of the rates quoted at approximately 11:00 a.m. New York City
time on such Interest Determination Date by three major banks in New York City,
selected by the Calculation Agent (after consultation with the Company), for
Three Month Deposits in Representative Amounts to leading European banks,
commencing on the second London Business Day immediately following such Interest
Determination Date; provided, however, that if fewer than three banks selected
by the Calculation Agent are quoting such rates, LIBOR for the applicable
Interest Period will be the same as LIBOR in effect for the immediately
preceding Interest Period.

      "Interest Determination Date" for an Interest Period means the second
London Business Day immediately preceding the first day of such Interest Period.

      "Interest Period" means the period commencing on an Interest Payment Date
(or commencing on the Original Issue Date, if no interest has been paid or duly
made available for payment since that date) and ending on the day before the
next succeeding Interest Payment Date or the Maturity Date, as the case may be.

      "LIBOR" for any Interest Determination Date will be the rate for deposits
in U.S. dollars having a maturity of three months for a period commencing on the
second London Business Day immediately following the Interest Determination Date
("Three Month Deposits"), as such rate appears on Telerate Page 3750 or a
successor reporter of such rates selected by the Calculation Agent and
acceptable to the Company, at approximately 11:00 a.m. London time on the
Interest Determination Date (the "Reported Rate"); provided, however, that in
certain circumstances described above, LIBOR will be determined in an
alternative manner.

      "London Business Day" means a day on which dealings in deposits in U.S.
dollars are transacted, or with respect to any future date, are expected to be
transacted, in the London interbank market.

      "Telerate Page 3750" means the display designated as Page 3750 on Bridge
Telerate, Inc., or any successor service, for the purpose of displaying London
interbank rates of major banks for U.S. dollar deposits.

      Upon request, the Calculation Agent will provide the current Interest Rate
and, if determined, the Interest Rate which will become effective for the next
Interest Period.

      All percentages resulting from any calculation of the Interest Rate will
be rounded, if necessary, to the nearest one-hundred thousandth of a percentage
point, with five one-millionths of a percentage point rounded upwards, and all
dollar amounts used in or resulting from such calculation will be rounded to the
nearest cent (with one-half cent being rounded upwards).

      Interest payments for this Note shall be computed for each Interest Period
on the basis of a year consisting of 360 days and paid for the actual number of
days for which interest is payable in that Interest Period. If any Interest
Payment Date falls on a day that is not a Business Day, the Interest Payment
Date will be the next succeeding Business Day. If the Maturity Date of this Note
falls on a day that is not a Business Day, the payment of principal and interest
will be made


                                      A-4
<Page>

on the next succeeding Business Day, and no interest on such payment shall
accrue for the period from and after the Maturity Date. This Note shall not be
redeemable prior to the Maturity Date.

      The Company, at its option, and subject to the terms and conditions
provided in the Indenture, will be discharged from any and all obligations in
respect of the Notes (except for certain obligations including obligations to
register the transfer or exchange of Notes, replace stolen, lost or mutilated
Notes, maintain paying agencies and hold monies for payment in trust, all as set
forth in the Indenture) if the Company deposits with the Trustee money, U.S.
Government Obligations which through the payment of interest thereon and
principal thereof in accordance with their terms will provide money, or a
combination of money and U.S. Government Obligations, in any event in an amount
sufficient, without reinvestment, to pay all the principal of and any premium
and interest on the Notes on the dates such payments are due in accordance with
the terms of the Notes.

      If an Event of Default shall occur and be continuing with respect to the
Notes, the principal of and interest on the Notes may be declared due and
payable in the manner and with the effect provided in the Indenture.

      The Indenture permits, with certain exceptions as therein provided, the
amendment thereof and the modifications of the rights and obligations of the
Company and the rights of the Noteholders under the Indenture at any time by the
Company and the Trustee with the consent of the Holders of a majority in
aggregate principal amount of the outstanding Notes. Any such consent or waiver
by the Holder of this Note shall be conclusive and binding upon such Holder and
upon all future Holders of this Note and of any Note issued upon the
registration of transfer hereof or in exchange therefor or in lieu thereof
whether or not notation of such consent or waiver is made upon this Note.

      As set forth in and subject to the provisions of the Indenture, no Holder
of any notes issued under the Indenture will have any right to institute any
proceeding with respect to the Indenture or for any remedy thereunder unless
such Holder shall have previously given to the Trustee written notice of a
continuing Event of Default with respect to such Notes, the Holders of a
majority in aggregate principal amount of the outstanding Notes of all series
under the Indenture in respect of which an Event of Default has occurred and is
continuing, considered as one class, shall have made written request and offered
reasonable indemnity to the Trustee to institute such proceeding as Trustee and
the Trustee shall have failed to institute such proceeding within 60 days;
PROVIDED, HOWEVER, that such limitations do not apply to a suit instituted by
the Holder hereof for the enforcement of payment of the principal of and any
premium or interest on this Note on or after the respective due dates expressed
here.

      No reference herein to the Indenture and to provisions of this Note or of
the Indenture shall alter or impair the obligation of the Company, which is
absolute and unconditional, to pay the principal of and any premium and interest
on this Note at the times, places and rates and the coin or currency prescribed
in the Indenture.

      As provided in the Indenture and subject to certain limitations therein
set forth, this Note may be transferred only as permitted by the legend hereto
and the provisions of the Indenture.


                                      A-5
<Page>

      The Indenture and the Notes shall be governed by, and construed in
accordance with, the laws of the State of New York without regard to conflicts
of law principles thereof.

      Unless the certificate of authentication hereon has been executed by the
Trustee, directly or through an Authenticating Agent by manual signature of an
authorized officer, this Note shall not be entitled to any benefit under the
Indenture or be valid or obligatory for any purpose.

      All terms used in this Note that are defined in the Indenture shall have
the meanings assigned to them in the Indenture unless otherwise indicated
herein.


                                      A-6
<Page>

      IN WITNESS WHEREOF, the Company has caused this instrument to be duly
executed.

                              AMEREN CORPORATION


                              By:
                                    ------------------------------------

                              Title:
                                      ----------------------------------

                              Attest:
                                      ----------------------------------

                              Title:
                                      ----------------------------------

TRUSTEE'S CERTIFICATE
OF AUTHENTICATION

Dated: December __, 2001

This Note is one of the Notes of the series herein designated, described or
provided for in the within- mentioned Indenture.

THE BANK OF NEW YORK, AS TRUSTEE

By:________________________________
        AUTHORIZED SIGNATORY




                                      A-7
<Page>

                                   SCHEDULE I

                      [144A][REGULATION S] GLOBAL SECURITY

The initial principal amount of Notes evidenced by this Global Note is
$150,000,000.

CHANGES TO PRINCIPAL AMOUNT OF NOTES EVIDENCED BY GLOBAL NOTE

<Table>
<Caption>
------------------------------- ---------------------------- ---------------------------- ----------------------------
                                 Principal Amount of Notes
                                 by which this Global Note
                                    is to be Reduced or      Remaining Principal Amount
                                 Increased, and Reason for     of Notes Represented by
             Date                  Reduction or Increase          this Global Note             Notation Made by
------------------------------- ---------------------------- ---------------------------- ----------------------------
------------------------------- ---------------------------- ---------------------------- ----------------------------
<S>                             <C>                          <C>                          <C>

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

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

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

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

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

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

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

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

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

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

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

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

------------------------------- ---------------------------- ---------------------------- ----------------------------
</Table>


                                      A-8
<Page>

                                   EXHIBIT A-1

                             CERTIFICATE OF TRANSFER

                               AMEREN CORPORATION

                    Floating Rate Notes due December 12, 2003

        FOR VALUE RECEIVED, the undersigned sells, assigns and transfers unto

PLEASE INSERT SOCIAL SECURITY OR OTHER
IDENTIFYING NUMBER OF ASSIGNEE

      Name and address of assignee must be printed or typewritten.
$
--------------------------------------------------------------------------------

principal amount of beneficial interest in the referenced Security of the
Company and does hereby irrevocably constitute and appoint

to transfer the said beneficial interest in such Security, with full power of
substitution in the premises.

The undersigned certifies that said beneficial interest in said Security is
being resold, pledged or otherwise transferred as follows:
(check one)

/ /   to the Company;

/ /   to a Person whom the undersigned reasonably believes is a qualified
      institutional buyer within the meaning of Rule 144A under the Securities
      Act of 1933, as amended (the "Securities Act"), purchasing for its own
      account or for the account of a qualified institutional buyer to whom
      notice is given that the resale, pledge or other transfer is being made in
      reliance on Rule 144A;

/ /   in an offshore transaction in accordance with Rule 904 of Regulation S
      under the Securities Act;

/ /   to an institution that is an "accredited investor" as defined in Rule
      501(a)(1), (2), (3) or (7) under the Securities Act that is acquiring this
      Security for investment purposes and not for distribution (attach a copy
      of an Accredited Investor Certificate in the form annexed signed by an
      authorized officer of the transferee);

/ /   as otherwise permitted by the non-registration legend appearing on this
      Security; or

/ /   as otherwise agreed by the Company, confirmed in writing to the Trustee,
      as follows: [describe]

Dated:_________________             Signature:________________________

                                    NOTICE: The signature to this assignment
                                    must correspond with the name as written
                                    upon the face of the within instrument in
                                    every particular without alteration or
                                    enlargement, or any change whatever.

                               SIGNATURE GUARANTEE

Signatures must be guaranteed by an "eligible guarantor institution" meeting the
requirement of the registrar, which requirements include membership or
participation in the Security Transfer Agent Medallion Program ("STAMP") or such
other "signature guarantee program" as may be determined by the registrar in
addition to, or in substitution for, STAMP, all in accordance with the
Securities Exchange Act of 1934, as amended.


                                     A-1-1
<Page>

             [FORM OF INSTITUTIONAL ACCREDITED INVESTOR CERTIFICATE]


[Transferor Name and Address]

Ladies and Gentlemen:

In connection with our proposed purchase of Floating Rate Notes due December 12,
2003 (the "Notes") issued by Ameren Corporation (the "Issuer"), we confirm that:

            1. We have received a copy of the Offering Memorandum (the "Offering
      Memorandum") relating to the Notes and such other information as we deem
      necessary in order to make our investment decision. We acknowledge that we
      have read and agree to the matters stated under the caption NOTICE TO
      INVESTORS in such Offering Memorandum, and the restrictions on duplication
      or circulation of, or disclosure relating to, such Offering Memorandum.

            2. We understand that any subsequent transfer of the Notes is
      subject to certain restrictions and conditions set forth in the Indenture
      relating to the Notes (the "Indenture") and that any subsequent transfer
      of the Notes is subject to certain restrictions and conditions set forth
      under NOTICE TO INVESTORS in the Offering Memorandum and the undersigned
      agrees to be bound by, and not to resell, pledge or otherwise transfer the
      Notes except in compliance with such restrictions and conditions and the
      Securities Act of 1933, as amended (the "Securities Act").

            3. We understand that the offer and sale of the Notes have not been
      registered under the Securities Act, and that the Notes may not be offered
      or sold except as permitted in the following sentence. We agree, on our
      own behalf and on behalf of any accounts for which we are acting as
      hereinafter stated, that if we sell any Notes, we will do so only (A) to
      the Company, (B) so long as the Notes are eligible for resale pursuant to
      Rule 144A under the Securities Act ("Rule 144A"), to a person whom we
      reasonably believe is a "qualified institutional buyer" within the meaning
      of Rule 144A that purchases for its own account or for the account of a
      qualified institutional buyer to whom notice is given that the resale,
      pledge or other transfer is being made in reliance on Rule 144A, (C) to an
      institutional "accredited investor" (as defined below) that, prior to such
      transfer, furnishes to the Trustee (as defined in the Indenture) a signed
      letter containing certain representations and agreements relating to the
      restrictions on transfer of the Notes (substantially in the form of this
      letter), (D) in an offshore transaction in accordance with Rule 904 of
      Regulation S under the Securities Act, (E) pursuant to the exemption from
      registration provided by Rule 144 under the Securities Act (if available),
      or (F) in accordance with another applicable exemption from the
      registration requirements of, or in a transaction not subject to, the
      Securities Act or pursuant to an effective registration statement under
      the Securities Act, and we further agree to provide to any person
      purchasing any of the Notes from us a notice advising such purchaser that
      resales of the Notes are restricted as stated herein.


                                      A-1-2
<Page>

            4. We understand that, on any proposed resale of any Notes, we will
      be required to furnish to the Trustee and the Issuer such certification
      and other information as the Trustee and the Issuer may reasonably require
      to confirm that the proposed sale complies with the foregoing
      restrictions. We further understand that the Notes purchased by us will
      bear a legend to the foregoing effect.

            5. We are an institutional "accredited investor" (as defined in Rule
      501(a)(1), (2), (3) or (7) of Regulation D under the Securities Act) and
      have such knowledge and experience in financial and business matters as to
      be capable of evaluating the merits and risks of our investment in the
      Notes, and we and any accounts for which we are acting are each able to
      bear the economic risk of our or its investment.

            6. We are acquiring the Notes purchased by us for our own account or
      for one or more accounts (each of which is an institutional "accredited
      investor") as to each of which we exercise sole investment discretion.

      You, the Issuer and the Trustee are entitled to rely upon this letter and
are irrevocably authorized to produce this letter or a copy hereof to any
interested party in any administrative or legal proceeding or official inquiry
with respect to the matters covered hereby.

                                          Very truly yours,


                                          By:___________________________
                                             Name:
                                             Title:




                                      A-1-3
<Page>

                                    EXHIBIT B

                             FORM OF DEFINITIVE NOTE

      THIS SECURITY HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS
AMENDED (THE "SECURITIES ACT"). THE HOLDER HEREOF, BY PURCHASING THIS SECURITY,
AGREES FOR THE BENEFIT OF THE COMPANY THAT THIS SECURITY MAY NOT BE RESOLD,
PLEDGED OR OTHERWISE TRANSFERRED PRIOR TO THE DATE WHICH IS TWO YEARS AFTER THE
LATER OF THE ORIGINAL ISSUANCE DATE THEREOF AND THE LAST DATE ON WHICH THE
COMPANY OR ANY AFFILIATE THEREOF WAS THE OWNER OF THIS SECURITY OR THE
EXPIRATION OF SUCH SHORTER PERIOD AS MAY BE PRESCRIBED BY RULE 144(k), OR ANY
SUCCESSOR PROVISION THEREOF, UNDER THE SECURITIES ACT (THE "RESALE RESTRICTION
TERMINATION DATE") OTHER THAN (1) TO THE COMPANY, (2) IN A TRANSACTION ENTITLED
TO AN EXEMPTION FROM REGISTRATION PROVIDED BY RULE 144 UNDER THE SECURITIES ACT,
(3) SO LONG AS THIS SECURITY IS ELIGIBLE FOR RESALE PURSUANT TO RULE 144A UNDER
THE SECURITIES ACT ("RULE 144A"), TO A PERSON WHOM THE SELLER REASONABLY
BELIEVES IS A QUALIFIED INSTITUTIONAL BUYER WITHIN THE MEANING OF RULE 144A
PURCHASING FOR ITS OWN ACCOUNT OR FOR THE ACCOUNT OF A QUALIFIED INSTITUTIONAL
BUYER TO WHOM NOTICE IS GIVEN THAT THE RESALE, PLEDGE OR OTHER TRANSFER IS BEING
MADE IN RELIANCE ON RULE 144A (AS INDICATED BY THE BOX CHECKED BY THE TRANSFEROR
ON THE CERTIFICATE OF TRANSFER ON THE REVERSE OF THIS SECURITY), (4) IN AN
OFFSHORE TRANSACTION IN ACCORDANCE WITH RULE 904 OF REGULATION S UNDER THE
SECURITIES ACT (AS INDICATED BY THE BOX CHECKED BY THE TRANSFEROR ON THE
CERTIFICATE OF TRANSFER ON THE REVERSE OF THIS SECURITY), (5) TO AN INSTITUTION
THAT IS AN "ACCREDITED INVESTOR" AS DEFINED IN RULE 501(a)(1), (2), (3) OR (7)
UNDER THE SECURITIES ACT (AS INDICATED BY THE BOX CHECKED BY THE TRANSFEROR ON
THE CERTIFICATE OF TRANSFER ON THE REVERSE OF THIS SECURITY) THAT IS ACQUIRING
THIS SECURITY FOR INVESTMENT PURPOSES AND NOT FOR DISTRIBUTION, AND A
CERTIFICATE IN THE FORM ATTACHED TO THIS SECURITY IS DELIVERED BY THE TRANSFEREE
TO THE COMPANY AND THE TRUSTEE OR (6) IN ACCORDANCE WITH ANOTHER APPLICABLE
EXEMPTION FROM, OR IN A TRANSACTION NOT SUBJECT TO, THE REGISTRATION
REQUIREMENTS OF THE SECURITIES ACT OR PURSUANT TO AN EFFECTIVE REGISTRATION
STATEMENT UNDER THE SECURITIES ACT, IN EACH CASE IN ACCORDANCE WITH ANY
APPLICABLE SECURITIES LAWS OF ANY STATE OF THE UNITED STATES. AN INSTITUTIONAL
ACCREDITED INVESTOR HOLDING THIS SECURITY AGREES IT WILL FURNISH TO THE COMPANY
AND THE TRUSTEE SUCH CERTIFICATES AND OTHER INFORMATION AS THEY MAY REASONABLY
REQUIRE TO CONFIRM THAT ANY TRANSFER BY IT OF THIS SECURITY COMPLIES WITH THE
FOREGOING RESTRICTIONS. THE FOREGOING RESTRICTIONS ON RESALE WILL NOT APPLY
SUBSEQUENT TO THE RESALE RESTRICTION TERMINATION DATE.


                                      B-1
<Page>

                               AMEREN CORPORATION
                    FLOATING RATE NOTE DUE DECEMBER 12, 2003

CUSIP/ISIN:                                     PRINCIPAL AMOUNT:

ORIGINAL ISSUE DATE: December 12, 2001          MATURITY DATE: December 12, 2003

INTEREST RATE: Floating, as set forth below     NUMBER:

      AMEREN CORPORATION, a corporation of the State of Missouri (the
"COMPANY"), for value received hereby promises to pay to ____________ or
registered assigns, the principal amount specified above on the Maturity Date
set forth above, and to pay interest thereon from and including the Original
Issue Date specified above or from and including the most recent date to which
interest has been paid or duly provided for, semi-annually in arrears on March
12, June 12, September 12 and December 12 in each year, commencing March 12,
2002, and on the Maturity Date, at the per annum Interest Rate determined by the
Calculation Agent on each Interest Determination Date, as such terms are defined
herein, until the principal hereof is paid or made available for payment. No
interest shall accrue on the Maturity Date, so long as the principal amount of
this Note is paid in full on the Maturity Date. The interest so payable and
punctually paid or duly provided for on any such Interest Payment Date will
(except for interest payable on the Maturity Date or, if applicable, upon
redemption or acceleration), as provided in the Indenture (as defined below), be
paid to the Person in whose name this Note is registered at the close of
business on the Regular Record Date for such interest, which shall be the
fifteenth calendar day , as the case may be, next preceding such Interest
Payment Date; provided that the first Interest Payment Date for any Note, the
Original Issue Date of which is after a Regular Record Date but prior to the
applicable Interest Payment Date, shall be the Interest Payment Date following
the next succeeding Regular Record Date; and provided, that interest payable on
the Maturity Date set forth above or, if applicable, upon redemption or
acceleration, shall be payable to the Person to whom principal shall be payable.
Except as otherwise provided in the Indenture (referred to on the reverse
hereof), any such interest not so punctually paid or duly provided for will
forthwith cease to be payable to the Holder on such Regular Record Date and
shall be paid to the Person in whose name this Note is registered at the close
of business on a Special Record Date for the payment of such defaulted interest
to be fixed by the Trustee, notice whereof shall be given to Noteholders not
more than fifteen days nor fewer than ten days prior to such Special Record
Date. Principal, applicable premium and interest due at the Maturity of this
Note shall be payable in immediately available funds when due upon presentation
and surrender of this Note at the corporate trust office of the Trustee or at
the authorized office of any paying agent in the Borough of Manhattan, The City
and State of New York or St. Louis, Missouri. Interest on this Note (other than
interest payable at Maturity) shall be paid by check payable in clearinghouse
funds to the Holder as its name appears on the register; provided, that if the
Trustee receives a written request from any Holder of Notes, the aggregate
principal amount of all of which having the same Interest Payment Date as this
Note equals or exceeds $10,000,000, on or before the applicable Regular Record
Date for such Interest Payment Date, interest on the Note shall be paid by wire
transfer of immediately available funds to a bank within the continental United
States (designated by such Holder in its request or by direct deposit into the
account of such Holder designated by such Holder in its request if such account
is maintained with the Trustee or any paying agent.


                                      B-2
<Page>

      REFERENCE IS HEREBY MADE TO THE FURTHER PROVISIONS OF THIS NOTE SET FORTH
IN FULL ON THE REVERSE HEREOF, WHICH FURTHER PROVISIONS SHALL FOR ALL PURPOSES
HAVE THE SAME EFFECT AS IF SET FORTH IN FULL AT THIS PLACE.

      Unless the certificate of authentication hereon has been executed by the
Trustee referred to on the reverse hereof, directly or through an Authenticating
Agent by manual signature of an authorized officer, this Note shall not be
entitled to any benefit under the Indenture or be valid or obligatory for any
purpose.

      IN WITNESS WHEREOF, the Company has caused this instrument to be duly
executed.

                              AMEREN CORPORATION



                              By:         _____________________________

                              Title:      _____________________________

                              Attest:     _____________________________

                              Title:      _____________________________

TRUSTEE'S CERTIFICATE
OF AUTHENTICATION

Dated: December __, 2001

This Note is one of the Notes of the series
herein designated, described or provided
for in the within-mentioned Indenture.

THE BANK OF NEW YORK, AS TRUSTEE

By:__________________________________
         AUTHORIZED SIGNATORY



                                      B-3
<Page>

                            [FORM OF REVERSE OF NOTE]
                               AMEREN CORPORATION
                    FLOATING RATE NOTE DUE DECEMBER 12, 2003

      This Note is one of a duly authorized issue of Floating Rate Notes due
December 12, 2003 (the "NOTES OF THIS SERIES") of the Company issued and to be
issued under an Indenture dated as of December 1, 2001, between the Company and
The Bank of New York, as trustee (herein called the "TRUSTEE", which term
includes any successor Trustee under the Indenture) and indentures supplemental
thereto (collectively, the "INDENTURE"). Under the Indenture, one or more series
of notes may be issued and, as used herein, the term "Notes" refers to the Notes
of this Series. Reference is hereby made to the Indenture for a more complete
statement of the respective rights, limitations of rights, duties and immunities
thereunder of the Company, the Trustee and the Noteholders and of the terms upon
which the Notes are and are to be authenticated and delivered. This Note is one
of the series designated on the face hereof, issued in the aggregate principal
amount of $150,000,000.

      Each Note of this Series shall be dated and issued as of the date of its
authentication by the Trustee and shall bear an Original Issue Date. Each Note
of this Series issued upon transfer, exchange or substitution of such Note shall
bear the Original Issue Date of such transferred, exchanged or substituted Note,
as the case may be.

      This Note will bear interest for each Interest Period at a per annum rate
("Interest Rate") determined by The Bank of New York, or its successor appointed
by the Company acting as calculation agent (the "Calculation Agent"), subject to
the maximum interest rate permitted by New York or other applicable state law,
as such law may be modified by United States law of general application. The
Interest Rate for each Interest Period will be equal to three month LIBOR on the
Interest Determination Date for such Interest Period plus 0.95%. Promptly upon
such determination, the Calculation Agent will notify the Trustee for the Notes,
if the Trustee is not then serving as the Calculation Agent, of the Interest
Rate for the new Interest Period. The Interest Rate determined by the
Calculation Agent, absent willful default, bad faith or manifest error, shall be
binding and conclusive upon the beneficial owners and holders of this Note, the
Company and the Trustee.

      Interest on this Note will accrue from and including the Original Issue
Date specified above to, but excluding, March 12, 2002, and thereafter, from and
including each Interest Payment Date to, but excluding, the next succeeding
Interest Payment Date or the Maturity Date, as the case may be.

      If the following circumstances exist on any Interest Determination Date,
the Calculation Agent shall determine LIBOR for the Notes as follows:

      (1) In the event no Reported Rate appears on Telerate Page 3750 as of
approximately 11:00 a.m. London time on an Interest Determination Date, the
Calculation Agent shall request the principal London offices of each of four
major banks in the London interbank market selected by the Calculation Agent
(after consultation with the Company) to provide a quotation of the rate at
which Three Month Deposits are offered by it to prime banks in the London
interbank market, as of approximately 11:00 a.m. London time on such Interest
Determination


                                      B-4
<Page>

Date, that is representative of single transactions at such time (the
"Representative Amounts"). If at least two rate quotations are provided, LIBOR
will be the arithmetic mean of the rate quotations obtained by the Calculation
Agent.

      (2) In the event no Reported Rate appears on Telerate Page 3750 the
Calculation Agent obtains fewer than two rate quotations, LIBOR will be the
arithmetic mean of the rates quoted at approximately 11:00 a.m. New York City
time on such Interest Determination Date by three major banks in New York City,
selected by the Calculation Agent (after consultation with the Company), for
Three Month Deposits in Representative Amounts to leading European banks,
commencing on the second London Business Day immediately following such Interest
Determination Date; provided, however, that if fewer than three banks selected
by the Calculation Agent are quoting such rates, LIBOR for the applicable
Interest Period will be the same as LIBOR in effect for the immediately
preceding Interest Period.

      "Interest Determination Date" for an Interest Period means the second
London Business Day immediately preceding the first day of such Interest Period.

      "Interest Period" means the period commencing on an Interest Payment Date
(or commencing on the Original Issue Date, if no interest has been paid or duly
made available for payment since that date) and ending on the day before the
next succeeding Interest Payment Date or the Maturity Date, as the case may be.

      "LIBOR" for any Interest Determination Date will be the rate for deposits
in U.S. dollars having a maturity of three months for a period commencing on the
second London Business Day immediately following the Interest Determination Date
("Three Month Deposits"), as such rate appears on Telerate Page 3750 or a
successor reporter of such rates selected by the Calculation Agent and
acceptable to the Company, at approximately 11:00 a.m. London time on the
Interest Determination Date (the "Reported Rate"); provided, however, that in
certain circumstances described above, LIBOR will be determined in an
alternative manner.

      "London Business Day" means a day on which dealings in deposits in U.S.
dollars are transacted, or with respect to any future date, are expected to be
transacted, in the London interbank market.

      "Telerate Page 3750" means the display designated as Page 3750 on Bridge
Telerate, Inc., or any successor service, for the purpose of displaying London
interbank rates of major banks for U.S. dollar deposits.

      Upon request, the Calculation Agent will provide the current Interest Rate
and, if determined, the Interest Rate which will become effective for the next
Interest Period.

      All percentages resulting from any calculation of the Interest Rate will
be rounded, if necessary, to the nearest one-hundred thousandth of a percentage
point, with five one-millionths of a percentage point rounded upwards, and all
dollar amounts used in or resulting from such calculation will be rounded to the
nearest cent (with one-half cent being rounded upwards).

      Interest payments for this Note shall be computed for each Interest Period
on the basis of a year consisting of 360 days and paid for the actual number of
days for which interest is payable


                                      B-5
<Page>

in that Interest Period. If any Interest Payment Date falls on a day that is not
a Business Day, the Interest Payment Date will be the next succeeding Business
Day. If the Maturity Date of this Note falls on a day that is not a Business
Day, the payment of principal and interest will be made on the next succeeding
Business Day, and no interest on such payment shall accrue for the period from
and after the Maturity Date. This Note shall not be redeemable prior to the
Maturity Date.

      The Company, at its option, and subject to the terms and conditions
provided in the Indenture, will be discharged from any and all obligations in
respect of the Notes (except for certain obligations including obligations to
register the transfer or exchange of Notes, replace stolen, lost or mutilated
Notes, maintain paying agencies and hold monies for payment in trust, all as set
forth in the Indenture) if the Company deposits with the Trustee money, U.S.
Government Obligations which through the payment of interest thereon and
principal thereof in accordance with their terms will provide money, or a
combination of money and U.S. Government Obligations, in any event in an amount
sufficient, without reinvestment, to pay all the principal of and any premium
and interest on the Notes on the dates such payments are due in accordance with
the terms of the Notes.

      If an Event of Default shall occur and be continuing with respect to the
Notes, the principal of and interest on the Notes may be declared due and
payable in the manner and with the effect provided in the Indenture.

      The Indenture permits, with certain exceptions as therein provided, the
amendment thereof and the modifications of the rights and obligations of the
Company and the rights of the Noteholders under the Indenture at any time by the
Company and the Trustee with the consent of the Holders of a majority in
aggregate principal amount of the outstanding Notes. Any such consent or waiver
by the Holder of this Note shall be conclusive and binding upon such Holder and
upon all future Holders of this Note and of any Note issued upon the
registration of transfer hereof or in exchange therefor in lieu thereof whether
or not notation of such consent or waiver is made upon the Note.

      As set forth in and subject to the provisions of the Indenture, no Holder
of any notes issued under the Indenture will have any right to institute any
proceeding with respect to the Indenture or for any remedy thereunder unless
such Holder shall have previously given to the Trustee written notice of a
continuing Event of Default with respect to such Notes, the Holders of a
majority in aggregate principal amount of the outstanding Notes of all series
under the Indenture in respect of which on Event of Default has occurred and is
continuing, considered as one class, shall have made written request and offered
reasonable indemnity to the Trustee to institute such proceeding as Trustee and
the Trustee shall have failed to institute such proceeding within 60 days;
provided, however, that such limitations do not apply to a suit instituted by
the Holder hereof for the enforcement of payment of the principal of and any
premium or interest on this Note on or after the respective due dates expressed
here.

      No reference herein to the Indenture and to provisions of this Note or of
the Indenture shall alter or impair the obligation of the Company, which is
absolute and unconditional, to pay the principal of and any premium and interest
on this Note at the times, places and rates and the coin or currency prescribed
in the Indenture.


                                      B-6
<Page>

      As provided in the Indenture and subject to certain limitations therein
set forth, the transfer of this Note is registrable in the Note register. Upon
surrender of this Note for registration or transfer at the corporate trust
office of the Trustee or such other office or agency as may be designated by the
Company in the Borough of Manhattan, the City and State of New York, or St.
Louis, Missouri, endorsed by or accompanied by a written instrument of transfer
in form satisfactory to the Company and the Note registrar, duly executed by the
Holder hereof or the attorney in fact of such Holder duly authorized in writing,
one or more new Notes of this Series of like tenor and of authorized
denominations and for the same aggregate principal amount will be issued to the
designated transferee or transferees.

      The Notes of this Series are issuable only in registered form, without
coupons, in denominations of $100,000 and integral multiples of $1,000 thereof.
As provided in the Indenture and subject to certain limitations therein set
forth, Notes of this Series are exchangeable for a like aggregate principal
amount of Notes of this Series of like tenor and of a different authorized
denomination, as requested by the Holder surrendering the same.

      No service charge shall be made for any such registration of transfer or
exchange but the Company may require payment of a sum sufficient to cover any
tax or other governmental charge payable in connection therewith.

      Prior to due presentment of this Note for registration of transfer, the
Company, the Trustee and any agent of the Company or the Trustee may treat the
Person in whose name this Note is registered as the owner thereof for all
purposes, whether or not this Note is overdue, and neither the Company, the
Trustee nor any such agent shall be affected by notice to the contrary.

      The Indenture and the Notes shall be governed by, and construed in
accordance with, the laws of the State of New York without regard to conflicts
of law principles thereof.

      All terms used in this Note that are defined in the Indenture shall have
the meanings assigned to them in the Indenture.



                                      B-7
<Page>

                                  ABBREVIATIONS

      The following abbreviations, when used in the inscription on the face of
this instrument, shall be construed as though they were written out in full
according to applicable laws or regulations:

TEN COM -- as tenants in common           UNIF GIFT
                                          MIN ACT -   _____ Custodian ______
                                                      (Cust)          (Minor)
TEN ENT -- as tenants by the
entireties                                Under Uniform Gifts to Minors

JT TEN -- as joint tenants with right
of survivorship and not as tenants in
common                                    ______________________________
                                                       State

                    Additional abbreviations may also be used
                         though not in the above list.

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



                                      B-8
<Page>

                             CERTIFICATE OF TRANSFER

                               AMEREN CORPORATION

                    Floating Rate Notes due December 12, 2003

        FOR VALUE RECEIVED, the undersigned sells, assigns and transfers unto

PLEASE INSERT SOCIAL SECURITY OR OTHER
IDENTIFYING NUMBER OF ASSIGNEE

      Name and address of assignee must be printed or typewritten.

$
--------------------------------------------------------------------------------

principal amount of beneficial interest in the referenced Security of the
Company and does hereby irrevocably constitute and appoint

to transfer the said beneficial interest in such Security, with full power of
substitution in the premises.

The undersigned certifies that said beneficial interest in said Security is
being resold, pledged or otherwise transferred as follows:
(check one)

/ /   to the Company;

/ /    to a Person whom the undersigned reasonably believes is a qualified
      institutional buyer within the meaning of Rule 144A under the Securities
      Act of 1933, as amended (the "Securities Act"), purchasing for its own
      account or for the account of a qualified institutional buyer to whom
      notice is given that the resale, pledge or other transfer is being made in
      reliance on Rule 144A;

/ /   in an offshore transaction in accordance with Rule 904 of Regulation S
      under the Securities Act;

/ /   to an institution that is an "accredited investor" as defined in Rule
      501(a)(1), (2), (3) or (7) under the Securities Act that is acquiring this
      Security for investment purposes and not for distribution (attach a copy
      of an Accredited Investor Certificate in the form annexed signed by an
      authorized officer of the transferee);

/ /   as otherwise permitted by the non-registration legend appearing on this
      Security; or

/ /   as otherwise agreed by the Company, confirmed in writing to the Trustee,
      as follows: [describe]

Dated:_________________             Signature:________________________

                                    NOTICE: The signature to this assignment
                                    must correspond with the name as written
                                    upon the face of the within instrument in
                                    every particular without alteration or
                                    enlargement, or any change whatever.

                               SIGNATURE GUARANTEE

Signatures must be guaranteed by an "eligible guarantor institution" meeting the
requirement of the registrar, which requirements include membership or
participation in the Security Transfer Agent Medallion Program ("STAMP") or such
other "signature guarantee program" as may be determined by the registrar in
addition to, or in substitution for, STAMP, all in accordance with the
Securities Exchange Act of 1934, as amended.



                                      B-9
<Page>

             [FORM OF INSTITUTIONAL ACCREDITED INVESTOR CERTIFICATE]


[Transferor Name and Address]

Ladies and Gentlemen:

In connection with our proposed purchase of Floating Rate Notes due December 12,
2003 (the "Notes") issued by Ameren Corporation (the "Issuer"), we confirm that:

            1. We have received a copy of the Offering Memorandum (the "Offering
      Memorandum") relating to the Notes and such other information as we deem
      necessary in order to make our investment decision. We acknowledge that we
      have read and agree to the matters stated under the caption NOTICE TO
      INVESTORS in such Offering Memorandum, and the restrictions on duplication
      or circulation of, or disclosure relating to, such Offering Memorandum.

            2. We understand that any subsequent transfer of the Notes is
      subject to certain restrictions and conditions set forth in the Indenture
      relating to the Notes (the "Indenture") and that any subsequent transfer
      of the Notes is subject to certain restrictions and conditions set forth
      under NOTICE TO INVESTORS in the Offering Memorandum and the undersigned
      agrees to be bound by, and not to resell, pledge or otherwise transfer the
      Notes except in compliance with such restrictions and conditions and the
      Securities Act of 1933, as amended (the "Securities Act").

            3. We understand that the offer and sale of the Notes have not been
      registered under the Securities Act, and that the Notes may not be offered
      or sold except as permitted in the following sentence. We agree, on our
      own behalf and on behalf of any accounts for which we are acting as
      hereinafter stated, that if we sell any Notes, we will do so only (A) to
      the Company, (B) so long as the Notes are eligible for resale pursuant to
      Rule 144A under the Securities Act ("Rule 144A"), to a person whom we
      reasonably believe is a "qualified institutional buyer" within the meaning
      of Rule 144A that purchases for its own account or for the account of a
      qualified institutional buyer to whom notice is given that the resale,
      pledge or other transfer is being made in reliance on Rule 144A, (C) to an
      institutional "accredited investor" (as defined below) that, prior to such
      transfer, furnishes to the Trustee (as defined in the Indenture) a signed
      letter containing certain representations and agreements relating to the
      restrictions on transfer of the Notes (substantially in the form of this
      letter), (D) in an offshore transaction in accordance with Rule 904 of
      Regulation S under the Securities Act, (E) pursuant to the exemption from
      registration provided by Rule 144 under the Securities Act (if available),
      or (F) in accordance with another applicable exemption from the
      registration requirements of, or in a transaction not subject to, the
      Securities Act or pursuant to an effective registration statement under
      the Securities Act, and we further agree to provide to any person
      purchasing any of the Notes from us a notice advising such purchaser that
      resales of the Notes are restricted as stated herein.


                                      B-10
<Page>

            4. We understand that, on any proposed resale of any Notes, we will
      be required to furnish to the Trustee and the Issuer such certification
      and other information as the Trustee and the Issuer may reasonably require
      to confirm that the proposed sale complies with the foregoing
      restrictions. We further understand that the Notes purchased by us will
      bear a legend to the foregoing effect.

            5. We are an institutional "accredited investor" (as defined in Rule
      501(a)(1), (2), (3) or (7) of Regulation D under the Securities Act) and
      have such knowledge and experience in financial and business matters as to
      be capable of evaluating the merits and risks of our investment in the
      Notes, and we and any accounts for which we are acting are each able to
      bear the economic risk of our or its investment.

            6. We are acquiring the Notes purchased by us for our own account or
      for one or more accounts (each of which is an institutional "accredited
      investor") as to each of which we exercise sole investment discretion.

      You, the Issuer and the Trustee are entitled to rely upon this letter and
are irrevocably authorized to produce this letter or a copy hereof to any
interested party in any administrative or legal proceeding or official inquiry
with respect to the matters covered hereby.

                                          Very truly yours,


                                          By:___________________________
                                             Name:
                                             Title:




                                      B-11