<PAGE>
                                                                   EXHIBIT 10.13

                 FIRST AMENDMENT TO TEXAS GENCO OPTION AGREEMENT



                  This First Amendment to Texas Genco Option Agreement (this
"Amendment"), entered into this 21st day of February, 2003, but effective as of
December 31, 2000, between CenterPoint Energy, Inc., a Texas corporation
("CenterPoint Energy" or "Regco"), and Reliant Resources, Inc., a Delaware
corporation ("Resources");

                  WHEREAS, Reliant Energy, Incorporated and Resources executed
that certain Texas Genco Option Agreement, dated as of December 31, 2000 (the
"Agreement");

                  WHEREAS, on August 31, 2002, CenterPoint Energy became the
successor to the rights and obligations of Reliant Energy, Incorporated under
the Agreement pursuant to the corporate restructuring of Reliant Energy,
Incorporated contemplated in that Agreement;

                  WHEREAS, CenterPoint Energy and Resources desire to amend the
Agreement in certain respects;

                  NOW THEREFORE, the parties, in consideration of the premises
and for good and valuable consideration agree as follow:

1.       Definitions.

         Capitalized terms used herein and not defined shall have the meaning
given in the Agreement.

2.       Amendment to Section 2.1.

         Section 2.1 of the Agreement, Organization of Genco LP, is amended by
deleting the reference therein to "December 31, 2001" and substituting therefor
"September 1, 2002."

3.       Amendment to Section 2.4.

         Section 2.4 of the Agreement, Genco Employee Matters, is amended by
deleting the reference therein to "January 1, 2002" and substituting therefor
"September 1, 2002."

4.       Amendment to Section 3.1.

         The penultimate sentence of the first paragraph of Section 3.1 shall be
amended to read: "The Control Premium Amount shall apply to the extent that the
PUCT includes up to a 10% control premium in the valuation of Texas Genco
pursuant to Section 39.262(h)(3) of the Utilities Code and shall equal the
proportional amount of the control premium applicable to the Shares.


<PAGE>


5.       Amendment to Section 3.2.

         Section 3.2 of the Agreement, Exercise of Option, is amended to read:

                  Resources may exercise the Option by giving written notice
         thereof to Regco during the Option Period. Subject to compliance with
         Section 3.3, and to satisfaction of the Regulatory Conditions to
         Exercise, delivery of and payment for the Shares (assuming the Option
         has been so exercised) shall be made at 10:00 A.M., Houston time, on
         the later of (a) the forty-fifth (45th) business day (or the next
         business day after the expiration of 45 days in the event the 45th day
         is a bank holiday) following the giving of such notice (or such other
         date as the parties agree) and (b) the first business day following the
         satisfaction of the Regulatory Conditions to Exercise (satisfaction of
         which shall be a condition precedent to such delivery and payment)
         (which date shall be the "Option Closing Date"), provided that if the
         amount of any Control Premium Amount included in the exercise price has
         not been determined by Final Order of the PUCT prior to the date for
         delivery and payment so determined, the payment made on the date so
         determined shall exclude any Control Premium Amount and such Control
         Premium Amount shall be paid in immediately available funds no later
         than 5 business days after the PUCT issues a Final Order determining
         market value under Section 39.262(h)(3) of the Utilities Code. Delivery
         of the Shares shall be made to Resources on the Option Closing Date
         against payment by Resources of the purchase price by wire transfer
         payable in same-day funds to the account specified by Regco. Delivery
         of the Shares shall be made by delivery to Resources of stock
         certificates representing the Shares, accompanied by appropriate stock
         powers or other instruments in proper form to effect such transfer. If
         Resources determines prior to the Option Period and within one year
         prior to the anticipated Option Closing Date that it intends in good
         faith, subject to economic conditions and other reasonable assumptions
         identified at such time, to exercise the Option, it and Regco shall
         make all appropriate regulatory filings, including filings under the
         Hart-Scott-Rodino Antitrust Improvements Act ("H-S-R") and the Nuclear
         Regulatory Commission, with a view to obtaining required approvals or
         expiration or termination of the applicable waiting period prior to the
         Option Exercise Date. Regco and Resources shall use commercially
         reasonable efforts to cause all other Regulatory Conditions to Exercise
         to be satisfied as promptly as practicable after the Option Exercise
         Date.

                  If Resources exercises the Option by giving notice as provided
         above, Resources shall be entitled to rescind its exercise of the
         Option by notice given to Regco on or before the 45th day following the
         Option Exercise Date if Resources has been unable by that date to
         secure financing for its purchase of the Shares on terms reasonably
         acceptable to Resources, despite the exercise by Resources of
         commercially reasonable efforts to obtain such financing. Upon the
         giving of such notice of rescission by Resources, the Option Period
         shall be deemed to have expired without the exercise of the Option.

                           If the Option Closing Date has not occurred by the
         last day of the sixteenth (16th) month after the month in which the
         Option Exercise Date occurs, the rights of the parties under this
         Agreement shall terminate.


<PAGE>


6.       Amendment of Section 3.6.

         Section 3.6 of the Agreement, Prohibitions on Market Activity, is
amended to read: "Prior to the later of (a) the Option Exercise Date; (b) if the
Option is not exercised, the Option Expiration Date; or (c) the conclusion of
the trading period for the Genco Stock as determined pursuant to Section
39.262(h)(3), neither Regco, Resources nor Genco shall, directly or indirectly
through any Subsidiary or other Person, purchase, sell, contract to purchase or
sell, or otherwise acquire or dispose of any shares of Genco Common Stock or any
options, warrants, rights, convertible securities or other securities
convertible into or exercisable or exchangeable for Common Stock, except as may
be necessary to effectuate this Agreement.

7.       Amendment to Section 6.1.

         Section 6.1 of the Agreement, Genco IPO or Genco Spinoff, is amended by
deleting the reference to "June 30, 2002" and substituting therefor "January 9,
2003."

8.       Amendment to Section 7.9.

         Section 7.9 of the Agreement, Dividends; No Repurchases of Capital
Stock, is deleted and the following Section 7.9 is substituted therefor:

         7.9 Dividends; No Repurchases of Capital Stock

                  For the period beginning on the Genco Public Ownership Date,
         the Genco Board of Directors shall establish a dividend policy under
         which it will distribute to its shareholders a dividend based on
         Genco's earnings and cash flows, subject to any limitations under
         corporate law or applicable regulatory restrictions, its financial
         condition and other factors the Board of Directors deems relevant. The
         dividend shall be set annually for each calendar year on or before
         December 31 of the immediately preceding calendar year. The initial
         annual dividend for 2003 will be $1.00. The dividend as established may
         be revised during any calendar year in the event the Genco Board of
         Directors reasonably concludes that circumstances would warrant a
         change or that an adjustment is required to the dividend to satisfy its
         obligations to the corporation. However, the dividend may only be
         increased by up to 10 percent once during any calendar year. Such
         dividend amount shall be paid through regular quarterly cash dividends
         complying with this Section 7.9 ("Regular Cash Dividends"). If the
         Option is exercised, Resources agrees that it will maintain this
         dividend policy so long as Resources owns less than 100% of Genco
         common stock.

                  If Resources exercises the Option, the purchase price for the
         Shares shall be adjusted for the difference between:

                  (a) the actual earnings of Genco per share from the Genco
         Public Ownership Date to the Option Closing Date (the Earnings Period),
         multiplied by the number of Shares, and

                  (b) the dividends per share paid by Genco to Regco during the
         Earnings Period multiplied by the Shares.


<PAGE>


                           To the extent dividends paid for each Share during
         the Earnings Period have been less than the actual per share earnings
         of Genco during the Earnings Period, the Option Price shall be adjusted
         upward for the difference; and to the extent dividends paid for each
         Share during the Earnings Period exceed the actual earnings of Genco
         per Share during the Earnings Period, the option price shall be
         credited with that difference. If and to the extent financial
         statements are not available to permit the earnings for the entire
         Earnings Period to be calculated at the date any such payment is to be
         made, the payment shall be made as promptly thereafter as practicable.
         Earnings for any portion of a month shall be calculated by pro rating
         earnings for the entire month based on the number of days in the
         applicable portion of such month.

                           From and after the Genco Public Ownership Date, Genco
         will not declare, set aside or pay any dividend payable in cash, stock
         or property, except for (a) Regular Cash Dividends, or (b) dividends
         payable solely in Genco Common Stock for which, if occurring during the
         Pricing Period, an adjustment is made pursuant to Section 3.1. Genco
         will not, and will not permit any of its Subsidiaries to, purchase or
         otherwise acquire for value any shares of Genco Common Stock.

9.       Amendment to Section 7.13.1.

         Section 7.13.1 is amended by deleting the reference therein to "15
days" and by substituting therefor "45 days."

10.      Amendment to Section 8.1.

         Section 8.1 of the Agreement, Board Composition, is amended by the
addition of the following at the end of the Section:

         If the Option is exercised, Regco agrees it will, subject to the
         satisfaction of the Regulatory Conditions to Exercise and the
         expiration of the 45-day rescission period set forth in Section 3.2,
         cause the shares of common stock of Genco owned or controlled by Regco
         to be voted in favor of nominees for Director proposed by Resources so
         that a majority of the Board of Directors as of the Option Closing Date
         will be nominees of Resources.

         11. No Implied Amendments. Except as specifically amended by this
Amendment, the Agreement shall remain in full force and effect in accordance
with its terms and is hereby ratified and confirmed.

         12. Counterparts. This Amendment may be executed in two or more
counterparts, each of which shall be deemed an original, and all of which shall
constitute one and the same instrument.


<PAGE>


                  IN WITNESS WHEREOF, the undersigned, by their officers
thereunto duly authorized, have executed this Amendment to the Texas Genco
Option Agreement as of the day and year first above written.

CENTERPOINT ENERGY, INC.                    RELIANT RESOURCES, INC.



By  /s/ David M. McClanahan                 By  /s/ Dan Hannon