<PAGE>

                                                                     EXHIBIT 4.2


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


                         FIRST SUPPLEMENTAL INDENTURE

                         dated as of November 1, 2000

                                      to

                                   INDENTURE

                         dated as of November 1, 2000

                       AMEREN ENERGY GENERATING COMPANY

                                      to

                       THE BANK OF NEW YORK, as Trustee


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


                                 $425,000,000

              $225,000,000  7.75% Senior Notes, Series A Due 2005
              $200,000,000  8.35% Senior Notes, Series B Due 2010
<PAGE>

     FIRST SUPPLEMENTAL INDENTURE (the "First Supplemental Indenture"), dated as
of November 1, 2000, to the Indenture, dated as of November 1, 2000 (the
"Original Indenture"), from AMEREN ENERGY GENERATING COMPANY, an Illinois
corporation (together with its successors and assigns, the "Issuer"), its
principal office and mailing address being at One Ameren Plaza, 1901 Chouteau
Avenue, P.O. Box 66149, St. Louis, Missouri 63166-6149, to THE BANK OF NEW YORK,
as trustee (the "Trustee"), its office and mailing address being at 101 Barclay
Street, New York, New York 10286.

                             W I T N E S S E T H:

     WHEREAS, the Issuer and the Trustee have heretofore executed and delivered
the Original Indenture to provide for the issuance from time to time of the
Issuer's Securities (as defined in the Original Indenture) to be issued in one
or more series;

     WHEREAS, Sections 2.1 and 7.1 of the Original Indenture provide, among
other things, that the Issuer and the Trustee may enter into indentures
supplemental to the Original Indenture for, among other things, the purpose of
establishing the designation, form, terms and provisions of Securities of any
series as permitted by Sections 2.1 and 7.1 of the Original Indenture;

     WHEREAS, the Issuer (i) desires the issuance of two (2) series of
Securities to be designated as hereinafter provided and (ii) has requested the
Trustee to enter into this First Supplemental Indenture for the purpose of
establishing the designation, form, terms and provisions of the Securities of
such series;

     WHEREAS, all action on the part of the Issuer necessary to authorize the
issuance of said Securities under the Original Indenture and this First
Supplemental Indenture (the Original Indenture, as supplemented by this First
Supplemental Indenture, being hereinafter called the "Indenture") has been duly
taken; and

     WHEREAS, all acts and things necessary to make said Securities, when
executed by the Issuer and authenticated and delivered by the Trustee as
provided in the Original Indenture, the legal, valid and binding obligations of
the Issuer, and to constitute these presents a valid and binding supplemental
indenture according to its terms, have been done and performed, and the
execution of this First Supplemental Indenture and the creation and issuance
under the Indenture of said Securities have in all respects been duly
authorized, and the Issuer, in the exercise of the legal right and power vested
in it, executes this First Supplemental Indenture and proposes to create,
execute, issue and deliver said Securities;

     NOW, THEREFORE, in order to establish the designation, form, terms and
provisions of, and to authorize the authentication and delivery of, said
Securities, and in consideration of the acceptance of said Securities by the
holders thereof and of other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, the parties hereto agree as
follows:

                                       1
<PAGE>

                                   ARTICLE I

                                  DEFINITIONS
                                  -----------

     Capitalized terms not otherwise defined herein shall have the meanings set
forth in the Original Indenture.

                                  ARTICLE II

                            THE TERMS OF THE NOTES
                            ----------------------

     Section 2.1 Terms of 7.75% Senior Notes, Series A due 2005 and 8.35% Senior
                 ---------------------------------------------------------------
Notes, Series B due 2010. (a) There is hereby created one (1) series of
------------------------
Securities designated: 7.75% Senior Notes, Series A due 2005, in the aggregate
principal amount of $225,000,000 (the " Series A Senior Notes"). Upon delivery
of a written order to the Trustee in accordance with the provisions of Section
2.1 of the Original Indenture, the Trustee shall authenticate and deliver the
Series A Senior Notes. Such written order shall specify the amount of the Series
A Senior Notes to be authenticated and the date on which such Series A Senior
Notes are to be authenticated.

          (b)  There is hereby created one (1) series of Securities designated:
8.35% Senior Notes, Series B due 2010, in the aggregate principal amount of
$200,000,000 (the "Series B Senior Notes" and collectively with the Series A
Senior Notes, the "Senior Notes"). Upon delivery of a written order to the
Trustee in accordance with the provisions of Section 2.1 of the Original
Indenture, the Trustee shall authenticate and deliver the Series B Senior Notes.
Such written order shall specify the amount of the Series B Senior Notes to be
authenticated and the date on which such Series B Senior Notes are to be
authenticated.

          (c)  The Senior Notes of each series shall be substantially in the
form of Exhibit A hereto.

     Section 2.2  Terms of Senior Notes Issued Hereunder in Global Form.
                  -----------------------------------------------------

          (a)  So long as DTC or its nominee is the registered owner or
Holder of a Global Security, DTC or its nominee, as the case may be, will be
considered the sole owner or Holder of the Senior Notes represented by such
Global Security for all purposes under the Original Indenture and under the
Senior Notes. No beneficial owner of an interest in a Global Security will be
able to transfer that interest except in accordance with DTC's applicable
procedures unless the Issuer shall issue certificates for the Senior Notes in
definitive registered form.

          (b)  All payments of the principal of, and interest and additional
amounts and premium, if any, on, a Global Security will be made to DTC or its
nominees, as the registered owners thereof.

          (c)  Transfers between participants in DTC will be effected in the
ordinary way in accordance with DTC rules and will be settled in same-day funds.

                                       2
<PAGE>

          (d)  Certificated definitive Senior Notes of a series may be in
denominations of less than $100,000 to the extent any redemption has reduced
such Holder's aggregate holding of such Senior Notes of the same series to less
than $100,000.

          (e)  If any redemption affecting the Senior Notes of a series would
result in the amount to be paid to a Holder of such affected Senior Note in
respect of such redemption not to equal $1,000 or an integral multiple thereof,
the Issuer shall instruct the Trustee to round the amount to be paid to such
Holder to the nearest $1,000 so that the amount to be paid to such Holder equals
$1,000 or an integral multiple thereof.

          (f)  Beneficial interests in a Global Security of a Series (and any
Securities issued in exchange therefor) will be subject to certain restrictions
on transfer set forth therein and in the Original Indenture and as set forth on
the form of such Global Security. Exhibit B, as referred to in Section 2.4 of
the Original Indenture and Exhibits C, D, E and F, as referred to in Section 2.6
of the Original Indenture are attached hereto as Exhibits B, C, D, E and F,
respectively.

          (g)  Except in the limited circumstances described under Section
2.2(h) below, beneficial interest in a Global Security will only be recorded by
book-entry and owners of beneficial interest in a Global Security will not be
entitled to receive physical delivery of certificates representing Securities.

          (h)  If (i) the Issuer notifies the Trustees in writing that DTC or
any successor depository is unwilling or unable to continue as a depository for
a Global Security or ceases to be a "clearing agency" registered under the
Exchange Act and a successor depository is not appointed by the Issuer within 90
days of such notice, (ii) the Issuer, at its option, notifies the Trustee in
writing that it elects to cause the issuance of the Securities issued hereunder
to be in certificated form or (iii) during an Event of Default, a holder of a
beneficial interest in a Global Security requests the issuance of certificated
Securities representing such holder's interest then, the Issuer shall issue
certificates for the Securities in definitive registered form substantially in
the form attached hereto in exchange for the Global Security outstanding.

          (i)  The holder of a certificated definitive registered Security may
transfer such Security in whole or in part by surrendering it at the Corporate
Trust Office of the Trustee in accordance with the terms of the Indenture and
such Security. Upon the transfer, exchange or replacement of definitive
Securities, the Issuer will deliver only definitive Securities that bear a
restrictive legend unless there is delivered to the Issuer such satisfactory
evidence, which may include an opinion of counsel, as may reasonably be required
by the Issuer, that neither the legend nor the restrictions on transfer set
forth therein are required to ensure compliance with the provisions of the
Securities Act.

     Section 2.3  Interest, Principal, Maturity Date and Regular Record Date.
                  ----------------------------------------------------------
Each Senior Note shall bear interest on the unpaid principal amount thereof from
time to time outstanding from the date of authentication thereof until such
amount is paid in full at the rate of interest set forth in the form of such
Senior Note attached hereto. The principal amount of each Senior Note shall be
due and payable at maturity as set forth in the form of Senior Note attached
hereto.

                                       3
<PAGE>

     Payment of principal of, premium, if any, and interest on the Senior Notes
of each series shall be made, as provided in Sections 2.4, 2.10, 3.2 and 3.4 of
the Original Indenture except that the final payment of principal of any series
of the Senior Notes shall be made on the due date therefor to the account of the
Holder as such account shall appear in the Security Register, which amount shall
be payable upon presentation and surrender of such Senior Note at the office of
the Issuer.

     Each Senior Note of a series shall mature on the date and in the amounts
set forth thereon.

     The record date applicable to the Senior Notes of each series issued
hereunder shall be as set forth in the form of Senior Note of such series
attached hereto.

     All payments of principal and interest with respect to certificated Senior
Notes will be made by bank check mailed on the interest payment date to the
address of such Holder on the Security Register or, for Holders of at least U.S.
$1,000,000 in aggregate principal amount of Senior Notes of a series, by wire
transfer on the interest payment date to a dollar account maintained by the
payee with a bank in The City of New York; provided, that a written request from
                                           --------
such Holder to such effect designating such account is received by the Trustee
and the Issuer or the paying agent no later than the record date immediately
preceding such Interest Payment Date.  Unless such designation is revoked, any
such designation made by such person with respect to such certificated Senior
Notes will remain in effect with respect to any future payments with respect to
such certificated Senior Note payable to such person.

     Section 2.4  Redemption.
                  ----------

          (a)  Optional Redemption.
               -------------------

          The Senior Notes of either series issued hereunder are subject to
optional redemption, in whole or in part, at any time at the option of the
Issuer at a redemption price equal to the outstanding principal amount of the
Senior Notes of such series being so redeemed plus accrued and unpaid interest
thereon to the date fixed for redemption together with the Applicable Premium
applicable thereto.

          (b)  Applicable Premium.
               ------------------

          As used herein, "Applicable Premium" means an amount calculated as of
the date (the "Determination Date") fixed for the redemption of the Senior Notes
of such series as follows:

          (i)  the average life of the remaining scheduled payments of principal
          in respect of Outstanding Senior Notes of such series (the "Remaining
                                                                      ---------
          Average Life") shall be calculated as of the Determination Date;
          ------------

          (ii) the yield to maturity calculated as of a date not more than five
          days prior to the Determination Date for the United States Treasury
          security having an average life equal to the Remaining Average Life of
          such series and trading in the secondary market at the price closest
          to the principal amount thereof (the "Primary Issue"); provided,
                                                -------------    --------
          however, that if no United States Treasury security has an average
          -------
          life equal to the Remaining Average Life of such series, the yields
          (the "Other Yields") for the two maturities of United States treasury
                ------------
          securities having average lives most closely corresponding to such
          Remaining Average Life

                                       4
<PAGE>

          and trading in the secondary market at the price closest to the
          principal amount thereof shall be calculated, and the yield to
          maturity for the Primary Issue shall be the yield interpolated or
          extrapolated from such Other Yields on a straight line basis, rounding
          in each of such relevant periods to the nearest month;

          (iii) the discounted present value of the then remaining scheduled
          payments of principal and interest (but excluding that portion of any
          scheduled payment of interest that is actually due and paid on the
          Determination Date) in respect of the Outstanding Senior Notes of such
          series shall be calculated as of the Determination Date using a
          discount factor equal to the sum of (x) the yield to maturity for the
          Primary Issue, plus (y) 25 basis points; and
                         ----

          (iv)  the amount of Applicable Premium in respect of the Senior Notes
          of such series to be redeemed shall be an amount equal to (x) the
          discounted present value of such Senior Notes to be redeemed
          determined in accordance with clause (iii) above, minus (y) the unpaid
                                                            -----
          principal amount of such Senior Notes; provided, however, that the
                                                 --------  -------
          Applicable Premium shall not be less than zero; and

          (v)   such calculation shall be made by an Investment Banker.

                                  ARTICLE III

                                 MISCELLANEOUS
                                 -------------

     Section 3.1  Execution of Supplemental Indenture.  This First Supplemental
                  -----------------------------------
Indenture is executed and shall be construed as an indenture supplemental to the
Original Indenture and, as provided in the Original Indenture, this First
Supplemental Indenture forms a part thereof.

     Section 3.2  Concerning the Trustee. The Trustee shall not be responsible
                  ----------------------
in any manner for or with respect to the validity or sufficiency of this First
Supplemental Indenture, or the due execution hereof by the Issuer, or for or
with respect to the recitals and statements contained herein, all of which
recitals and statements are made solely by the Issuer.

     Section 3.3  Counterparts. This First Supplemental Indenture may be
                  ------------
executed in any number of counterparts, each of which when so executed shall be
deemed to be an original; but all such counterparts shall together constitute
but one and the same instrument.

     Section 3.4  GOVERNING LAW. THIS FIRST SUPPLEMENTAL INDENTURE AND EACH
                  -------------
SENIOR NOTE ISSUED HEREUNDER SHALL, PURSUANT TO SECTION 5-1401 OF THE NEW YORK
GENERAL OBLIGATIONS LAW, BE GOVERNED BY THE LAW OF THE STATE OF NY, WITHOUT
REFERENCE TO THE CHOICE OF LAW PROVISIONS THEREOF (OTHER THAN SUCH SECTION 5-
1401).

                                       5
<PAGE>

     IN WITNESS WHEREOF, the parties hereto have caused this First Supplemental
Indenture to be duly executed as of November 1, 2000.


                               AMEREN ENERGY GENERATING COMPANY,
                               as Issuer


                               By:        /s/ Gary L. Rainwater
                                   ----------------------------------------
                                   Name:  Gary L. Rainwater
                                   Title: President


                               THE BANK OF NEW YORK, as Trustee


                               By:        /s/ Albert Lundy
                                   ----------------------------------------
                                   Name:  Albert Lundy
                                   Title: Qualified Agent

<PAGE>

                                                                       EXHIBIT A

                               FORM OF SECURITY

     [INCLUDE IF SECURITY IS A GLOBAL SECURITY DEPOSITED WITH THE U.S.
DEPOSITARY -- UNLESS THIS CERTIFICATE IS PRESENTED BY AN AUTHORIZED
REPRESENTATIVE OF THE DEPOSITORY TRUST COMPANY TO THE ISSUER OR ITS AGENT FOR
REGISTRATION OF TRANSFER, EXCHANGE OR PAYMENT, AND ANY CERTIFICATE ISSUED IN
EXCHANGE FOR THIS CERTIFICATE OR ANY PORTION HEREOF IS REGISTERED IN THE NAME OF
CEDE & CO. OR TO SUCH OTHER ENTITY AS IS REQUESTED BY AN AUTHORIZED
REPRESENTATIVE OF THE DEPOSITORY TRUST COMPANY OR SUCH OTHER NAME AS IS
REQUESTED BY AN AUTHORIZED REPRESENTATIVE OF THE DEPOSITORY TRUST COMPANY (AND
ANY PAYMENT HEREON IS MADE TO CEDE & CO.), ANY TRANSFER, PLEDGE OR OTHER USE
HEREOF FOR VALUE OR OTHERWISE BY OR TO ANY PERSON OTHER THAN THE DEPOSITORY
TRUST COMPANY OR A NOMINEE THEREOF IS WRONGFUL INASMUCH AS THE REGISTERED OWNER
HEREOF, CEDE & CO., HAS AN INTEREST HEREIN.

     TRANSFERS OF THIS GLOBAL SECURITY SHALL BE LIMITED TO TRANSFERS IN WHOLE,
BUT NOT IN PART, TO NOMINEES OF THE DEPOSITORY TRUST COMPANY OR TO A SUCCESSOR
THEREOF OR SUCH SUCCESSOR'S NOMINEE AND TRANSFERS OF PORTIONS OF THIS GLOBAL
SECURITY SHALL BE LIMITED TO TRANSFERS MADE IN ACCORDANCE WITH THE RESTRICTIONS
SET FORTH IN SECTION 2.6 OF THE INDENTURE REFERRED TO ON THE REVERSE HEREOF.]

     THIS NOTE (OR ITS PREDECESSOR) WAS ORIGINALLY ISSUED IN A TRANSACTION
EXEMPT FROM REGISTRATION UNDER THE UNITED STATES SECURITIES ACT OF 1933, AS
AMENDED (THE "SECURITIES ACT"), AND MAY NOT BE OFFERED, SOLD, PLEDGED OR
OTHERWISE TRANSFERRED IN THE ABSENCE OF SUCH REGISTRATION OR AN APPLICABLE
EXEMPTION THEREFROM AND IN ANY EVENT MAY BE SOLD OR OTHERWISE TRANSFERRED ONLY
IN ACCORDANCE WITH THE INDENTURE, COPIES OF WHICH ARE AVAILABLE FOR INSPECTION
AT THE CORPORATE TRUST OFFICE OF THE TRUSTEE IN NEW YORK.

     EACH PURCHASER OF THIS NOTE IS HEREBY NOTIFIED THAT THE SELLER MAY BE
RELYING ON THE EXEMPTION FROM THE PROVISIONS OF SECTION 5 OF THE SECURITIES ACT
PROVIDED BY RULE 144A THEREUNDER. EACH HOLDER OF THIS NOTE REPRESENTS TO AMEREN
ENERGY GENERATING COMPANY THAT (a) SUCH HOLDER WILL NOT SELL, PLEDGE OR
OTHERWISE TRANSFER THIS NOTE (WITHOUT THE CONSENT OF AMEREN ENERGY GENERATING
COMPANY) OTHER THAN (i) TO A QUALIFIED INSTITUTIONAL BUYER IN A TRANSACTION
COMPLYING WITH RULE 144A UNDER THE SECURITIES ACT, (ii) IN ACCORDANCE WITH RULE
144 UNDER THE SECURITIES ACT, (iii) OUTSIDE THE UNITED STATES IN A TRANSACTION
MEETING THE REQUIREMENTS OF REGULATION S UNDER THE SECURITIES ACT, (iv) PURSUANT
TO ANOTHER AVAILABLE EXEMPTION FROM REGISTRATION UNDER THE SECURITIES ACT,
SUBJECT, IN THE CASE OF

<PAGE>

CLAUSES (ii), (iii) OR (iv), TO THE RECEIPT BY AMEREN ENERGY GENERATING COMPANY
OF AN OPINION OF COUNSEL OR SUCH OTHER EVIDENCE ACCEPTABLE TO AMEREN ENERGY
GENERATING COMPANY THAT SUCH RESALE, PLEDGE OR TRANSFER IS EXEMPT FROM THE
REGISTRATION REQUIREMENTS OF THE SECURITIES ACT OR (v) PURSUANT TO AN EFFECTIVE
REGISTRATION STATEMENT AND THAT (b) THE HOLDER WILL, AND EACH SUBSEQUENT HOLDER
IS REQUIRED TO, NOTIFY ANY PURCHASER OF THIS NOTE OF THE RESALE RESTRICTIONS
REFERRED TO HEREIN AND DELIVER TO THE TRANSFEREE (OTHER THAN A QUALIFIED
INSTITUTIONAL BUYER) PRIOR TO THE SALE A COPY OF THE TRANSFER RESTRICTIONS
APPLICABLE HERETO (COPIES OF WHICH MAY BE OBTAINED FROM THE TRUSTEE).

                         [FORM OF FACE OF SENIOR NOTE]

                           CUSIP [      ][   ][   ]

                                 [Common Code]
                               [ISIN][        ]

No.

                                       $
                       AMEREN ENERGY GENERATING COMPANY
                          * % Senior Notes Due 20 **


     Ameren Energy Generating Company (the "Issuer"), for value received hereby
                                            ------
promises to pay to ___________________ or registered assigns the principal sum
of ____________ Dollars at the Issuer's office or agency for said purpose as
provided in the Indenture referred to herein, on November 1,**      in such coin
or currency of the United States of America as at the time of payment shall be
legal tender for the payment of public and private debts, and to pay interest,
semi-annually in arrears on May 1 and November 1 of each year, commencing May 1,
2001, on said principal sum in like coin or currency at the rate per annum set
forth above at said offices or agencies from the date of original issuance or
the most recent interest payment date to which interest on the Senior Notes has
been paid or duly provided for.  Notwithstanding the foregoing, if the date
hereof is after April 15 or October 15, as the case may be, and before the
following May 1 or November 1, this Senior Note shall bear interest from such
May 1 or November 1; provided, that if the Issuer shall default in the payment
of interest due on such May 1 or November 1, then this Senior Note shall bear
interest from the next preceding May 1 or November 1 to which interest on the
Senior Notes has been paid or duly provided for.  The interest so payable on any
May 1 or November 1 will, except as otherwise provided in the Indenture referred
to on the reverse hereof, be paid to the Person in whose name this Senior Note
is registered at the close of business on the 15th day of April or the 15th day
of October preceding such May 1 or November 1, whether or not such day is a
Business Day; provided, that

____________________

*    Insert 7.75% or 8.35% as applicable.

**   Insert 2005 or 2010 as applicable.

<PAGE>

principal, premium, if any, and interest shall be paid by mailing on the
interest payment date a check for such to or upon the written order of the
registered Holders of Senior Notes entitled thereto at their last address as it
appears on the Senior Notes Register or, upon written application to the Trustee
by a Holder of $1,000,000 or more in aggregate principal amount of Senior Notes,
by wire transfer on the interest payment date of immediately available funds to
an account maintained by such Holder with a bank or other financial institution.
Interest on this Senior Note shall be computed on the basis of a 360-day year
comprised of twelve 30-day months. Additional Interest shall accrue on this
Senior Note under the circumstances provided for in the Registration Rights
Agreement.

     Interest on overdue principal and (to the extent permitted by applicable
law) on overdue installments of interest (including without limitation during
the 5-day period referred to in Section 4.1(b) of the Indenture) shall accrue at
the rate per annum set forth above.

     Reference is made to the further provisions set forth on the reverse
hereof. Such further provisions shall for all purposes have the same effect as
though fully set forth at this place.

<PAGE>

     This Senior Note shall not be entitled to any benefit under the Indenture,
or be valid or obligatory, until the certificate of authentication hereon shall
have been duly signed by the Trustee acting under the Indenture.

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

                                        THE BANK OF NEW YORK, as Trustee



                                        By: ______________________________
                                            Name:
                                            Title:
(SEAL)
Attested:



By: ______________________________
Name:
Title:

<PAGE>

                       [FORM OF REVERSE OF SENIOR NOTE]

                       AMEREN ENERGY GENERATING COMPANY

                          * % Senior Notes Due 20  **


     This Senior Note is one of a duly authorized issue of debt securities of
the Issuer, limited to the aggregate principal amount of $          ***
(except as otherwise provided in the Indenture mentioned below), issued or to be
issued pursuant to an Indenture dated as of November 1, 2000 as supplemented by
the First Supplemental Indenture dated as of November 1, 2000 (as so
supplemented, the "Indenture"), duly executed and delivered by the Issuer to the
                   ---------
Trustee.  Reference is hereby made to the Indenture and all indentures
supplemental thereto for a description of the rights, limitations of rights,
obligations, duties and immunities thereunder of the Trustee, the Issuer and the
Holders (the words "Holders" or "Holder" meaning the registered holders or
                    -------      ------
registered holder) of the Senior Notes.  Capitalized terms used herein, but not
otherwise defined herein, shall have the meanings assigned to them in the
Indenture.

     In case an Event of Default shall have occurred and be continuing, the
principal of all the Securities may be declared due and payable, in the manner
and with the effect, and subject to the conditions, provided in the Indenture.
The Indenture provides that in certain events such declaration and its
consequences may be waived by the Holders of a majority in aggregate principal
amount of the Securities then Outstanding and that, prior to any such
declaration, such Holders may waive any past default under the Indenture and its
consequences except a default in the payment of principal of or premium, if any,
or interest on any of the Securities and as otherwise provided in the Indenture.
Any such consent or waiver by the Holder of this Senior Note (unless revoked as
provided in the Indenture) shall be conclusive and binding upon such Holder and
upon all future Holders and owners of this Senior Note and any Security which
may be issued in exchange or substitution hereof, whether or not any notation
thereof is made upon this Senior Note or such other Security.

     The Indenture permits the Issuer and the Trustee, with the consent of the
Holders of not less than a majority in aggregate principal amount of the
Securities of all series at the time Outstanding considered as one class,
evidenced as in the Indenture provided, to modify the Indenture or any
supplemental indentures or the rights of the Holders of the Senior Notes;
provided that no such modification shall (a) change the Stated Maturity of the
--------
principal of, or any installment of principal of or interest on, any Security,
or reduce the principal amount thereof, or reduce the rate or extend the time of
payment of interest thereon, or reduce any amount payable on redemption thereof
or impair or affect the right of any Holder of the Security to institute suit
for the payment thereof without the consent of the Holder of each Security so
affected; or (b)(i) reduce the aforesaid percentage of Securities, the consent
of the Holders of which is required for any such modification or the percentage
of Securities, the consent of Holders of

________________
*    Insert 7.75% or 8.35% as applicable.

**   Insert 2005 or 2010 as applicable.

***  Insert $225,000,000 or $200,000,000 as applicable.
<PAGE>

which is required for any waiver provided for in the Indenture; (ii) change any
obligation of the Issuer to maintain an office or agency for payment of and
transfer and exchange of the Securities; or (iii) make certain changes to
provisions relating to waiver or to the provision for supplementing the
Indenture; in each case without the consent of the Holders of all Securities
then Outstanding.

     No reference herein to the Indenture and no provision of this Senior Note
or of the Indenture shall alter or impair the obligation of the Issuer, which is
absolute and unconditional, to pay the principal of and premium, if any, and
interest on this Senior Note at the place, times, and rate, and in the currency,
herein prescribed.

     The Senior Notes are issuable only as registered Senior Notes without
coupons in denominations of $100,000 and any integral multiple of $1,000 in
excess thereof.

     At the office or agency of the Issuer referred to on the face hereof and in
the manner and subject to the limitations provided in the Indenture, Senior
Notes may be presented for exchange for a like aggregate principal amount of
Senior Notes of other authorized denominations.

     Upon due presentment for registration of transfer of this Senior Note at
the above-mentioned office or agency of the Issuer, a new Senior Note or Senior
Notes of authorized denominations, for a like aggregate principal amount, will
be issued to the transferee as provided in the Indenture.  No service charge
shall be made for any such transfer, but the Issuer may require payment of a sum
sufficient to cover any tax or other governmental charge that may be imposed in
relation thereto.

     The Senior Notes may be redeemed in whole or in part (if in part, by lot or
by such other method as the Trustee shall deem fair or appropriate) prior to
Stated Maturity at the option of the Issuer, upon mailing a notice of such
redemption not less than 30 nor more than 60 days prior to the date fixed for
redemption to the Holders of Senior Notes, all as provided in the Indenture, at
a redemption price equal to the principal amount thereof plus accrued and unpaid
interest thereon, if any, to the date of redemption, plus the Applicable
Premium.

     Subject to payment by the Issuer of a sum sufficient to pay the amount due
on redemption, interest on this Senior Note shall cease to accrue upon the date
duly fixed for redemption of this Senior Note.

     The Issuer, the Trustee, and any authorized agent of the Issuer or the
Trustee, may deem and treat the registered Holder hereof as the absolute owner
of this Senior Note (whether or not this Senior Note shall be overdue and
notwithstanding any notation of ownership or other writing hereon made by anyone
other than the Issuer or the Trustee or any authorized agent of the Issuer or
the Trustee), for the purpose of receiving payment of, or on account of, the
principal hereof and premium, if any, and, subject to the provisions on the face
hereof, interest hereon and for all other purposes, and neither the Issuer nor
the Trustee nor any authorized agent of the Issuer or the Trustee shall be
affected by any notice to the contrary.

     No recourse shall be had for the payment of the principal of, or premium,
if any, or the interest on this Senior Note, for any claim based hereon, or
otherwise in respect hereof, or based

                                       2
<PAGE>

on or in respect of the Indenture or any indenture supplemental thereto, against
any incorporator, shareholder, officer or director, as such, past, present or
future, of the Issuer or of any successor corporation, either directly or
through the Issuer or any successor corporation, whether by virtue of any
constitution, statute or rule of law or by the enforcement of any assessment or
penalty or otherwise, all such liability being, by the acceptance hereof and as
part of the consideration for the issue hereof, expressly waived and released.

                                       3
<PAGE>

               [FORM OF TRUSTEE'S CERTIFICATE OF AUTHENTICATION]

Dated:

     This is one of the Senior Notes referred to in the within-mentioned
Indenture.

                                                THE BANK OF NEW YORK, as Trustee


                                                By: ____________________________
                                                          Authorized Signatory

                                       4
<PAGE>

                             [FORM OF ASSIGNMENT]

I or we assign and transfer this Security to:

             (Insert assignee's social security or tax I.D. number)


_______________________________________________________
_______________________________________________________
_______________________________________________________
_______________________________________________________
(Print or type name, address and zip code of assignee)

and irrevocably appoint:


_______________________________________________________
_______________________________________________________

Agent to transfer this Security on the books of the Issuer.  The Agent may
substitute another to act for him.

Date: _____________________       Your Signature: ___________________


                                   (Sign exactly as your name
                                   appears on the other side of
                                   this Security)

               *Signature Guarantee:________________________


               *Signatures must be guaranteed by an "eligible guarantor
          institution" meeting the requirements of the Registrar, which
          requirements include membership or participation in 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.

                                       5
<PAGE>

                                                                       EXHIBIT B

                     FORM OF RESTRICTED PERIOD CERTIFICATE
                       AMEREN ENERGY GENERATING COMPANY

                                    [DATE]

The Depository Trust Company
[Address]

The Bank of New York,
 as Trustee
[Attn: Corporate Trust
101 Barclay Street
New York, New York 10286]

     Re:  Ameren Energy Generating Company        % Senior Notes Due 20__
          ---------------------------------------------------------------

Ladies and Gentlemen:

     Reference is hereby made to the Indenture dated as of November 1, 2000 (the
"Indenture") from Ameren Energy Generating Company, to The Bank of New York, as
 ---------
Trustee.  Capitalized terms used and not defined herein shall have the meanings
given them in the Indenture.

     This letter is related to U.S. $______________ principal amount of Senior
Notes represented by the Restricted Regulation S Global Security, held by the
Trustee pursuant to Section 2.4 of the Indenture.  We hereby certify that the
offering of the Senior Notes has closed and that the restricted period (as
defined in Regulation S) with respect to the offer and sale of the Senior Notes
has terminated.

                                                THE BANK OF NEW YORK, as Trustee



                                                By: ____________________________
                                                         Authorized Signatory

cc:    [Euroclear]
       [CLEARSTREAM]
<PAGE>

                                                                       EXHIBIT C

                         FORM OF TRANSFER CERTIFICATE
            FOR TRANSFER OR EXCHANGE FROM RULE 144A GLOBAL SECURITY
       OR IAI GLOBAL SECURITY TO RESTRICTED REGULATION S GLOBAL SECURITY

                      (Transfers or exchanges pursuant to
                     Section 2.6(b)(ii) of the Indenture)

The Bank of New York,
as Trustee
[Attn: Corporate Trust
101 Barclay Street
New York, New York 10286]

     Re:  Ameren Energy Generating Company      % Senior Notes
          Due 20    (the "Senior Notes")
          ------------------------------------------------------

     Reference is hereby made to the Indenture dated as of November 1, 2000 (the
"Indenture") from Ameren Energy Generating Company to The Bank of New York, as
 ---------
Trustee.  Capitalized terms used but not defined herein shall have the meanings
given to them in the Indenture.

     This letter relates to U.S.$______________ principal amount of Senior Notes
which are held in the form of the [Rule 144A] [IAI] Global Security (CUSIP No.[
][           ]) with the U.S. Depositary in the name of [insert name of
transferor] (the "Transferor").  The Transferor has requested a transfer or
                  ----------
exchange of such beneficial interest for an interest in the Restricted
Regulation S Global Security (CUSIP No.            ) to be held with [Euroclear]
[Clearstream] (Common Code ________) through the U.S. Depositary.

     In connection with such request and in respect of such Senior Notes, the
Transferor does hereby certify that such transfer or exchange has been effected
in accordance with the transfer restrictions set forth in the Indenture and the
Senior Notes and pursuant to and in accordance with Regulation S under the
Securities Act, and accordingly the Transferor does hereby certify that:

     (1) the offer of the Senior Notes was not made to a person in the United
States;

     [(2)  at the time the buy order was originated, the transferee was outside
the United States or the Transferor and any person acting on its behalf
reasonably believed that the transferee was outside the United States,]*

     [(2)  the transaction was executed in, on or through the facilities of a
designated offshore securities market and neither the Transferor nor any person
acting on its behalf knows that the transaction was pre-arranged with a buyer in
the United States,]*

     (3) no directed selling efforts have been made in contravention of the
requirements of Rule 903(b) or 904(b) of Regulation S, as applicable, and
<PAGE>

     (4) the transaction is not part of a plan or scheme to evade the
registration requirements of the Securities Act.

     This certificate and the statements contained herein are made for your
benefit and the benefit of the Issuer and the Initial Purchasers.

                                                [Insert Name of Transferor]


                                                By:_____________________________
                                                   Name:
                                                   Title:

Dated:____________, 2000


cc:  Ameren Energy Generating Company

___________________
*  Insert one of these two provisions, which come from the definition of
"offshore transactions" in Regulation S.

                                       2
<PAGE>

                                                                       EXHIBIT D

                         FORM OF TRANSFER CERTIFICATE
            FOR TRANSFER OR EXCHANGE FROM RULE 144A GLOBAL SECURITY
      OR IAI GLOBAL SECURITY TO UNRESTRICTED REGULATION S GLOBAL SECURITY

                      (Exchanges or transfers pursuant to
                     Section 2.6(b)(iii)of the Indenture)

The Bank of New York,
as Trustee
[Attn: Corporate Trust
101 Barclay Street
New York, New York  10286]

     Re:  Ameren Energy Generating Company      % Senior Notes
     Due 20    (the "Senior Notes")
     -------------------------------------------------------------

     Reference is hereby made to the Indenture dated as of November 1, 2000 (the
"Indenture") from Ameren Energy Generating Company to The Bank of New York, as
 ---------
Trustee.  Capitalized terms used but not defined herein shall have the meanings
given to them in the Indenture.

     This letter relates to U.S.$______________ principal amount of Senior Notes
which are held in the form of the [Rule 144A] [IAI] Global Security (CUSIP No. [
][            ]) with the U.S. Depositary in the name of [insert name of
transferor] (the "Transferor").  The Transferor has requested an exchange or
                  ----------
transfer of such beneficial interest in the Senior Notes for an interest in the
Unrestricted Regulation S Global Security (CUSIP No.            ).

     In connection with such request and in respect of such Senior Notes, the
Transferor does hereby certify that such exchange or transfer has been effected
in accordance with the transfer restrictions set forth in the Indenture and the
Senior Notes and,

     (i) with respect to transfers made in reliance on Regulation S under the
Securities Act, the Transferor does hereby certify that:

     (1) the offer of the Senior Notes was not made to a person in the United
States;

     [(2)  at the time the buy order was originated, the transferee was outside
the United States or the Transferor and any person acting on its behalf
reasonably believed that the transferee was outside the United States,]*

     [(2)  the transaction was executed in, on or through the facilities of a
designated offshore securities market and neither the Transferor nor any person
acting on its behalf knows that the transaction was pre-arranged with a buyer in
the United States,]*

     (3) no directed selling efforts have been made in contravention of the
requirements of Rule 903(b) or 904(b) of Regulation S, as applicable, and
<PAGE>

     (4)   the transaction is pursuant to and in accordance with Regulation S
and is not part of a plan or scheme to evade the registration requirements of
the United States Securities Act of 1933, as amended (the "Securities Act");

     (ii)  with respect to transfers made in reliance on Rule 144 under the
Securities Act, the transferor does hereby certify that the Senior Notes are
being transferred in a transaction permitted by Rule 144 under the Securities
Act,

     (iii) with respect to transfers made in reliance on another exemption from
the Securities Act (including without limitation Rule 144A), the transferor does
hereby certify that the following is the basis for the exemption:
_________________, or

     (iv)  with respect to an exchange, the transferor does hereby certify that
either (x) the Security being exchanged is not a "restricted security" as
defined in Rule 144 under the Securities Act or (y) the exchange is being made
to facilitate a contemporaneous transfer that complies with Section 2.6(b)(iii)
of the Indenture.

     This certificate and the statements contained herein are made for your
benefit and the benefit of the Issuer and the Initial Purchasers.

                                                [Insert Name of Transferor]



                                                By:_____________________________
                                                   Name:
                                                   Title:
Dated:_______________, 2000


cc:  Ameren Energy Generating Company

______________________
*  Insert one of these two provisions, which come from the definition of
"offshore transactions" in Regulation S.

                                       2
<PAGE>

                                                                       EXHIBIT E

             FORM OF TRANSFER CERTIFICATE FOR TRANSFER OR EXCHANGE
          FROM RESTRICTED REGULATION S GLOBAL SECURITY, UNRESTRICTED
                        REGULATION S GLOBAL SECURITY OR
               IAI GLOBAL SECURITY TO RULE 144A GLOBAL SECURITY

                      (Exchanges or transfers pursuant to
                     Section 2.6(b)(iv) of the Indenture)

The Bank of New York,
as Trustee
[Attn: Corporate Trust
101 Barclay Street
New York, New York 10286]

     Re:  Ameren Energy Generating Company      % Senior Notes
     Due 20    (the "Senior Notes")
     ------------------------------------------------------------

     Reference is hereby made to the Indenture dated as of November 1, 2000 (the
"Indenture") from Ameren Energy Generating Company to The Bank of New York, as
 ---------
Trustee.  Capitalized terms used but not defined herein shall have the meanings
given to them in the Indenture.

     This letter relates to U.S. $______________ principal amount of Senior
Notes which are held in the form of the [[Restricted][Unrestricted] Regulation S
Global Security (CUSIP No.            ) with [Euroclear] [Clearstream].*
(Common Code _______)] [IAI Global Security (CUSIP No.            )] through the
U.S. Depositary in the name of [insert name of transferor] (the "Transferor").
                                                                 ----------
The Transferor has requested a transfer or exchange of such beneficial interest
in the Senior Notes for an interest in the Rule 144A Global Security.

     In connection with such request, and in respect of such Senior Notes, the
Transferor does hereby certify that such Senior Notes are being transferred or
exchanged in accordance with (i) the transfer restrictions set forth in the
Senior Notes and (ii) Rule 144A under the Securities Act to a transferee that
the Transferor reasonably believes is purchasing the Senior Notes for its own
account or an account with respect to which the transferee exercises sole
investment discretion and the transferee and any such account is a "qualified
institutional buyer" within the meaning of Rule 144A, in each case in a
transaction meeting the requirements of Rule 144A and in accordance with any
applicable securities laws of any state of the United States or any other
jurisdiction.

___________________
*  Select appropriate depositary.
<PAGE>

     This certificate and the statements contained herein are made for your
benefit and the benefit of the Issuer and the Initial Purchasers.

                                                [Insert Name of Transferor]



                                                By:_____________________________
                                                   Name:
                                                   Title:

Dated:___________, 2000

cc:  Ameren Energy Generating Company
<PAGE>

                                                                       EXHIBIT F

                   FORM OF TRANSFER CERTIFICATE FOR TRANSFER
       OR EXCHANGE FROM RULE 144A GLOBAL SECURITY, RESTRICTED REGULATION
         GLOBAL SECURITY OR UNRESTRICTED REGULATION S GLOBAL SECURITY
                            TO IAI GLOBAL SECURITY

                      (Exchanges or transfers Pursuant to
                      Section 2.6(b)(v) of the Indenture)

The Bank of New York,
as Trustee
[Attn: Corporate Trust
101 Barclay Street
New York, New York 10286]

     Re:  Ameren Energy Generating Company      % Senior Notes
     Due 20    (the "Senior Notes")
     ------------------------------------------------------------

     Reference is hereby made to the Indenture dated as of November 1, 2000 (the
"Indenture") from Ameren Energy Generating Company to The Bank of New York, as
 ---------
Trustee.  Capitalized terms used but not defined herein shall have the meanings
given to them in the Indenture.

     This letter relates to U.S.$______________ principal amount of Senior Notes
which are held in the form of the [Rule 144A Global Security (CUSIP No.
) with the U.S. Depositary] [[Restricted][Unrestricted] Regulation S Global
Security (CUSIP No.           ) with [Euroclear] [Clearstream]]* in the name of
[insert name of transferor] (the "Transferor").  The Transferor has requested an
                                  ----------
exchange or transfer of such Global Security for an interest in the IAI Global
Security (CUSIP No.           ).

     In connection with such request with respect to a transfer or exchange for
an interest in the IAI Global Security, the Transferor does hereby certify that
such exchange or transfer has been effected in accordance with the transfer
restrictions set forth in the Rule 144A Global Security, the Restricted
Regulation S Global Security and the Unrestricted Regulation S Global Security
(as the case may be) and, with respect to transfers made in reliance on Rule 144
under the Securities Act, certify that the Securities are being transferred in a
transaction permitted by Rule 144 under the Securities Act.

________________
*  Select appropriate depositary.
<PAGE>

     In connection with such request with respect to a transfer or exchange for
an interest in the IAI Global Security, and in respect of such Rule 144A Global
Security, Restricted Regulations S Global Security and Unrestricted Regulation S
Global Security (as the case may be), the Transferor does hereby certify that
such Global Security is being transferred or exchanged in accordance with (i)
the transfer restrictions set forth in the Global Securities and (ii) in
accordance with the Securities Act to a transferee that the Transferor
reasonably believes is purchasing the IAI Global Securities for its own account
or an account with respect to which the transferee exercises sole investment
discretion and the transferee and any such account is an institution that is an
"accredited investor" within the meaning of Rule 501(a)(1), (2), (3) or (7)
under the Securities Act, in each case purchasing IAI Global Securities in a
transaction exempt from the Securities Act and in accordance with any applicable
securities laws of any state of the United States or any other jurisdiction.

     This certificate and the statements contained herein are made for your
benefit and the benefit of the Issuer and the Initial Purchasers.


                                                [Insert Name of Transferor]



                                                By:_____________________________
                                                   Name:
                                                   Title:

Dated:_______________, 2000

cc:  Ameren Energy Generating Company