Exhibit 2.1


                               AMENDMENT NO. 1 TO
                            STOCK PURCHASE AGREEMENT


         THIS AMENDMENT NO. 1, dated as of March 23, 2004, to the STOCK PURCHASE
AGREEMENT, dated as of February 2, 2004, is entered into by and among Ameren
Corporation, a Missouri corporation ("Purchaser"), Illinova Corporation, an
Illinois corporation ("Seller"), Illinova Generating Company, an Illinois
corporation ("IGC"), and Dynegy Inc., an Illinois corporation ("Dynegy").
Dynegy, IGC and Seller are referred to herein as the "Dynegy Parties".

                              W I T N E S S E T H:

         WHEREAS, Purchaser and the Dynegy Parties entered into a Stock Purchase
Agreement, dated February 2, 2004 (the "Original Agreement"), providing for the
sale to Purchaser of all of the capital stock of Illinois Power Company, an
Illinois corporation, held by Seller, and IGC's 20% share of Electric Energy,
Inc., an Illinois corporation; and

         WHEREAS, Purchaser and the Dynegy Parties wish to amend the Original
Agreement as set forth herein;

         NOW, THEREFORE, in consideration of the premises and the mutual terms,
conditions and agreements set forth herein, the parties hereto hereby agree as
follows:

                  Section 1 Defined Terms. All capitalized terms used and not
         defined herein have the meanings set forth in the Original Agreement.

                  Section 2  Amendments to Section 1.1.

                           A. The definition of "Ancillary Agreements" included
                  in Section 1.1 of the Original Agreement is amended to add the
                  following after the reference to the "Tier 2 Memorandum,":

                           "the Interim PPA Rider,".

                           B. Section 1.1 of the Original Agreement is amended
                  to add the following definition after the definition of
                  Intercompany Note:

                           ""Interim PPA Rider" means the agreement between DMG
                  and IPC in the form of Exhibit I, with such changes as may be
                  required by Governmental Authorities as a condition to
                  approving the transactions or any portion thereof contemplated
                  by this Agreement and the Ancillary Agreements that are
                  required to be accepted by Seller or by Purchaser, pursuant to
                  the provisions of Section 5.3 or are otherwise accepted by
                  Seller and by Purchaser.".

                  Section 3 Amendment to Section 5.21. Section 5.21(b) of the
         Original Agreement is amended by changing the reference to "30 days" in
         the second sentence to "90 days".



                  Section 4 Amendment to Section 8.2(j). Section 8.2(j) of the
         Original Agreement is amended by deleting such Section and replacing it
         with the following:

                  "Solely if the Closing occurs after September 10, 2004, the
                   Tilton Assets, or IPC's rights, interests, assets,
                   liabilities and obligations with respect to the electric
                   generating equipment and real estate at the Tilton Energy
                   Center, shall have been transferred to DMG and IPC shall have
                   no remaining obligations with respect to the Tilton Assets.
                   For avoidance of doubt, it is the intent of this Section
                   8.2(j) that all of IPC's rights, interest, assets,
                   liabilities and obligations with respect to the Tilton
                   Assets, or IPC's rights, interests, assets, liabilities and
                   obligations with respect to the electric generating equipment
                   and real estate at the Tilton Energy Center, shall have been
                   transferred to or otherwise come to be held by DMG, excluding
                   only those rights, interests, assets, liabilities and
                   obligations of IPC as a public utility that are necessary for
                   the continued operation by DMG of the Tilton Energy Center,
                   including IPC's rights, interests, assets, liabilities and
                   obligations under the Interconnection Agreement and the gas
                   service contracts listed on Schedule 3.19. Nothing herein
                   shall preclude DMG from acquiring the Tilton Assets, or the
                   electric generating equipment and real estate at the Tilton
                   Energy Center, at any time prior to September 10, 2004."

                  Section 5 Amendment to Schedule 5.3(b). Schedule 5.3(b) to the
         Original Agreement is amended as set forth in Exhibit 1 hereto.

                   Section 6 Amendment to Schedule 5.15. Schedule 5.15 to the
          Original Agreement is amended as set forth in Exhibit 2 hereto.

                   Section 7 Amendment to Schedule 8.1(b). Schedule 8.1(b) to
          the Original Agreement is amended as set forth in Exhibit 3 hereto.

                  Section 8 Amendment to Schedule 8.2(b). Schedule 8.2(b) to the
         Original Agreement is amended as set forth in Exhibit 4 hereto.

                  Section 9 Amendment to Exhibit B. Exhibit B to the Original
         Agreement is amended by changing the reference to "thirty (30) days" in
         the bracketed note at the top of page 1 of Exhibit B to "ninety (90)
         days".

                  Section 10 Amendment to Exhibit F. Exhibit F to the Original
         Agreement is amended by replacing the agreement attached as Exhibit F
         with the Form of Blackstart Agreement attached hereto as Exhibit 5.

                  Section 11 Addition of Exhibit I. The Original Agreement is
         amended by adding, immediately following Exhibit H, a new Exhibit I
         containing the form of Interim PPA Rider attached hereto as Exhibit 6.

                  Section 12 No Other Amendments. Except as set forth herein,
         the Original Agreement remains in full force and effect.


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                  Section 13 Counterparts. This Agreement may be executed in one
         or more counterparts, and by the parties in separate counterparts, each
         of which when executed shall be deemed to be an original but all of
         which taken together shall constitute one and the same agreement.















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              IN WITNESS WHEREOF, Seller, IGC, Dynegy and Purchaser have caused
     this Amendment No. 1 to the Original Agreement to be executed as of the
     date first written above by their respective officers thereunto duly
     authorized.

                                           ILLINOVA CORPORATION


                                           By: /s/ Robert T. Ray
                                             --------------------------------
                                             Name:  Robert T. Ray
                                             Title: Senior Vice President &
                                                    Treasurer

                                           ILLINOVA GENERATING COMPANY


                                           By: /s/ Robert T. Ray
                                             --------------------------------
                                             Name:  Robert T. Ray
                                             Title: Senior Vice President &
                                                    Treasurer

                                           DYNEGY INC.


                                           By: /s/ Robert T. Ray
                                             --------------------------------
                                             Name:  Robert T. Ray
                                             Title: Senior Vice President &
                                                    Treasurer

                                           AMEREN CORPORATION

                                           By: /s/ Steven R. Sullivan
                                             --------------------------------
                                             Name:  Steven R. Sullivan
                                             Title: Senior Vice President,
                                                    Regulatory Policy, General
                                                    Counsel & Secretary











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