SUPPLEMENTAL
INDENTURE





THIS SUPPLEMENTAL INDENTURE, dated
as of March 31, 2008, between POTOMAC ELECTRIC POWER
COMPANY, a corporation duly organized and existing under the laws of the
District of Columbia and the Commonwealth of Virginia, having its principal
office at 701 Ninth Street, N.W., Washington D.C. 20068 (herein called the
“Company”), and THE BANK OF NEW YORK, a New
York banking corporation, having its principal corporate trust office at 101
Barclay Street, New York, New York 10286, as trustee (herein called the “Trustee”).  Except
as otherwise defined or unless the context otherwise requires, capitalized terms
used in this Supplemental Indenture and defined in the Indenture (as hereinafter
defined) shall have the meanings specified in the Indenture.





RECITALS
OF THE COMPANY





WHEREAS, the Company and the Trustee
are parties to that certain Indenture, dated as of November 17, 2003 (the “Indenture”), providing for
the issuance from time to time of its Securities in an unlimited aggregate
principal amount to be issued in one or more series as contemplated
therein.





WHEREAS, the Company and the Trustee
desire to amend Section 1302(c) of the Indenture to permit any series of
Securities to be secured by more than one series of First Mortgage Bonds,
pursuant to the terms and subject to the conditions set forth
herein.





WHEREAS, pursuant to Section 1201(d)
and (k) of the Indenture, the Company and the Trustee are permitted, without the
consent of any Holders, to enter into an indenture supplemental to the Indenture
to change any provision of the Indenture, provided such change does not
adversely affect the interest of the Holders of Securities of any series or
Tranche in any material respect.





WHEREAS, the Company has determined
that the amendment to Section 1302(c) of the Indenture, as provided for in this
Supplemental Indenture, does not adversely affect the interest of the Holders of
Securities of any series or Tranche in any material respect.





WHEREAS, the execution and delivery of
this Supplemental Indenture has been duly authorized and all acts conditions
necessary to make this Supplemental Indenture a valid agreement of the Company
have been performed.





NOW, THEREFORE, THIS INDENTURE
WITNESSETH:





For and in consideration of the
premises and other good and valuable consideration, the receipt whereof is
hereby acknowledged, it is mutually covenanted and agreed, for the equal and
proportionate benefit of all Holders of the Securities or any series thereof, as
follows:








ARTICLE
ONE





AMENDMENT





Section 1.1.   Section
1302(c) of the Indenture is hereby amended and restated in its entirety as
follows:





(c)           Any
series of First Mortgage Bonds may secure the payment of the principal, premium,
if any, and interest on only one corresponding series of
Securities.  Any series of Securities may be secured by one or more
series of First Mortgage Bonds, provided that, in the circumstance where more
than one series of First Mortgage Bonds secures the payment of the principal,
premium, if any, and interest on a series of Securities, (i) the aggregate
principal amount of First Mortgage Bonds of such series of First Mortgage Bonds
securing such series of Securities collectively shall equal or exceed the
outstanding aggregate principal amount of such series of Securities and (ii) the
terms of each series of First Mortgage Bonds securing such series of Securities
shall satisfy the conditions of Section 1302(a)(ii) through
(vi).  Each Security of a series shall specify each of the one or more
series of First Mortgage Bonds that secures the Securities of such
series.








ARTICLE
TWO





MISCELLANEOUS
PROVISIONS





Section 2.1.  Except as
specifically amended and supplemented by this Supplemental Indenture, the
Indenture shall remain in full force and effect and is hereby ratified and
confirmed, and the Indenture and this Supplemental Indenture shall together
constitute one and the same instrument.





Section 2.2.  This
Supplemental Indenture shall be governed by and construed in accordance with the
laws of the State of New York, excluding any conflicts or choice of law rule or
principle that might otherwise refer construction or interpretation of this
Supplemental Indenture to the substantive law of another
jurisdiction.





Section 2.3.  All covenants
and agreements in this Supplemental Indenture by the Company and Trustee shall
bind their respective successors and assigns, whether so expressed or
not.





Section 2.4.  The Trustee
accepts the amendment of the Indenture as hereby effected but only upon the
terms and conditions set forth in the Indenture, as amended and supplemented by
this Supplemental Indenture.





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Section 2.5.  This
Supplemental Indenture may be executed in any number of counterparts, each of
which so executed shall be deemed to be an original, but all such counterparts
shall together constitute but one and the same instrument.





Section 2.6.  The Trustee
accepts the amendment of the Indenture effected by this Supplemental Indenture
and agrees to execute the trust created by the Indenture as hereby amended, but
only upon the terms and conditions set forth in the Indenture, including the
terms and provisions defining and limiting the liabilities and responsibilities
of the Trustee, which terms and provisions shall in like manner define and limit
its liabilities and responsibilities in the performance of the trust created by
the Indenture as hereby amended.  Without limiting the generality of
the foregoing, the Trustee shall not be responsible in any manner whatsoever for
or with respect to any of the recitals contained herein, all of which recitals
are made solely by the Company, or for or with respect to (i) the validity or
sufficiency of this Supplemental Indenture or any of the terms or provisions
hereof, (ii) the due authorization hereof by the Company by corporate action or
otherwise, or (iii) the due execution hereof by the Company, and the Trustee
makes no representation with respect to any such matters.





[Signature page
follows]




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IN WITNESS WHEREOF, the parties hereto
have caused this Supplemental Indenture to be duly executed, all as of the day
and year first above written.





POTOMAC
      ELECTRIC POWER COMPANYBy:
            /s/
      A. J. KAMERICKName:
            Anthony
      J. KamerickTitle:
            Vice
      President and TreasurerTHE
      BANK OF NEW YORK, as TrusteeBy:
            /s/
      CHERYL L. CLARKEName:
            Cheryl
      L. ClarkeTitle:
            Vice
      President





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