MARKETSPAN CORPORATION
                           175 EAST OLD COUNTRY ROAD
                          HICKSVILLE, NEW YORK 11801

                                                                     EXHIBIT 5


                                                      May 22, 1998


MarketSpan Corporation
175 East Old Country Road
Hicksville, New York 11801

            Re:   INVESTOR PROGRAM

Gentlemen:

     As Senior Vice  President and General  Counsel for  MarketSpan  Corporation
(the  "Company"),  I am familiar  with the  proposal of the Company to issue and
sell shares of its Common Stock par value $0.01 per share (the  "Common  Stock")
pursuant to an Investor Program (the "Plan"). In connection with the proceedings
before the Securities and Exchange  Commission  with respect  thereto,  I submit
this  opinion  and  hereby  consent  to its use as  Exhibit  5 to the  Company's
Registration Statement on Form S-3 (the "Registration Statement") proposed to be
filed by the Company under the  Securities  Act of 1933, as amended,  and to the
use of my name in said Registration  Statement and the Prospectus forming a part
thereof (the "Prospectus").

      I am familiar with the  Certificate  of  Incorporation  and By-Laws of the
Company,  as well as the Registration  Statement and the Prospectus  relating to
the Plan.

      Based upon the foregoing and upon my general  familiarity with the affairs
of the Company, I advise you that in my opinion:

     1. The Company is a corporation  duly organized and validly  existing under
the laws of the State of New York.

     2. No state regulatory body or agency has jurisdiction over the transaction
proposed by the Company or any part thereof.

      3. No federal  commission or agency other than the Securities and Exchange
Commission,  under the Securities Act of 1933, as amended, has jurisdiction over
the transaction proposed by the Company or any part thereof.

      4. All action  necessary to make valid the issuance and sale of the Common
Stock will have been taken when (a) the Registration Statement shall have become
effective;  (b)  the  Board  of  Directors  of  the  Company  shall  have  taken
appropriate  action to approve and authorize the issuance and sale of the Common
Stock on the terms set forth in the Registration Statement;





MARKETSPAN CORPORATION
May 22, 1998
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and (c) the Company  shall have  received the full  consideration  therefor (not
less than $0.01 per share).

      5. When the  foregoing  steps shall have been taken,  the Common Stock (a)
will conform in all substantial  respects to the description of the Common Stock
contained  in the  Registration  Statement  and (b) will be legally  and validly
issued, fully paid and nonassessable.

     6. The  offering of the Common  Stock is not subject to  preemptive  rights
under the laws of the State of New York.

                                Very truly yours,

                                /s/ Leonard P. Novello

                                Leonard P. Novello
                                Senior Vice President and
                                General Counsel