<PAGE>

               UNITED STATES SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549
                                  FORM 10-K/A
                                AMENDMENT NO. 1

[X] ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(D) OF THE SECURITIES
    EXCHANGE ACT OF 1934
    FOR THE FISCAL YEAR ENDED DECEMBER 31, 1999
                                      OR
[_] TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(D) OF THE SECURITIES
    EXCHANGE ACT OF 1934
    FOR THE TRANSITION PERIOD FROM___________TO____________.


                        Commission file number 1-14768
                        ------------------------------

                                     NSTAR
                                     -----
             (Exact name of registrant as specified in its charter)

       MASSACHUSETTS                                         04-3466300
       -------------                                         ----------
(STATE OR OTHER JURISDICTION                               (I.R.S. EMPLOYER
OF INCORPORATION OR ORGANIZATION)                          IDENTIFICATION NO.)

800 BOYLSTON ST., BOSTON MASSACHUSETTS                           02199
-------------------------------------------------------        ----------
(ADDRESS OF PRINCIPAL EXECUTIVE OFFICES)                       (ZIP CODE)

REGISTRANT'S TELEPHONE NUMBER, INCLUDING AREA CODE: 617-424-2000
                                                    ------------

SECURITIES REGISTERED PURSUANT TO SECTION 12(B) OF THE ACT:


                                                         NAME OF EACH EXCHANGE
                                                         ---------------------
          TITLE OF EACH CLASS                             ON WHICH REGISTERED
          -------------------                             -------------------
 Common shares, par value $1 per share                  New York Stock Exchange
                                                         Boston Stock Exchange

SECURITIES REGISTERED PURSUANT TO SECTION 12(G) OF THE ACT: NONE

  Indicate by check mark whether the registrant (1) has filed all reports
required to be filed by Section 13 or 15(d) of the Securities Exchange Act of
1934 during the preceding 12 months (or for such shorter period that the
registrant was required to file such reports), and (2) has been subject to such
filing requirements for the past 90 days. YES  [X]   NO  [_]

  Indicate by check mark if disclosure of delinquent filers pursuant to Item 405
of Regulation S-K is not contained herein, and will not be contained, to the
best of registrant's knowledge, in definitive proxy or information statements
incorporated by reference in Part III of this Form 10-K or any amendment to this
Form 10-K  [ X]

  The aggregate market value of the voting stock held by non-affiliates of the
registrant as of March 15, 2000 computed as the average of the high and low
market price of the common stock as reported in the listing of composite
transactions for New York Stock Exchange listed securities in the Wall Street
Journal: $2,209,524,613.

  Indicate the number of shares outstanding of each for the registrant's classes
of common stock, as of the latest practicable date.

<TABLE>
<S>                                                <C>
                        CLASS                       0UTSTANDING AT MARCH 15, 2000
                        -----                       -----------------------------
             COMMON SHARES $1 PER VALUE                 56,836,646 SHARES

         DOCUMENTS INCORPORATED BY REFERENCE              PART IN FORM 10-K
        ------------------------------------              ------------------
 Portions of the Registrant's Notice of 2000 Annual      Parts I, II and III
     Meeting, Proxy Statement and 1999 Financial
          Information Dated March 30, 2000
             (pages as specified herein)
</TABLE>
<PAGE>

                                     NOTICE

This Form 10-K/A, dated September 29, 2000, amends NSTAR's 1999 Form 10-K as
filed on March 30, 2000. Certain additional material contracts, as listed within
Part IV, Item 14, "Exhibits, Financial Statement Schedules and Reports on Form
8-K", and the Consent of Independent Accountants are being filed in this
Amendment No. 1. Item 14 is provided in its entirety and should be read in
conjunction with NSTAR's 1999 Form 10-K.

                                       2
<PAGE>

                                    PART IV

Item 14.   Exhibits, Financial Statement Schedules and Reports on Form 8-K

  (a) The following documents are filed as part of this Form 10-K:


1. Financial Statements:

<TABLE>
<CAPTION>
                                                                                                            Page
                                                                                                           -------
<S>                                                                                                        <C>
Consolidated Statements of Income for the years ended December 31, 1999, 1998 and 1997.                         34

Consolidated Statements of Retained Earnings for the years ended December 31, 1999, 1998 and 1997.              35

Consolidated Balance Sheets as of December 31, 1999 and 1998.                                                   36

Consolidated Statements of Cash Flows for the years ended December 31, 1999, 1998 and 1997.                     37

Notes to Consolidated Financial Statements.                                                                     38

Selected Consolidated Quarterly Financial Data (Unaudited)                                                      17

Report of Independent Accountants.                                                                              81

2.  Financial Statement Schedules:

Schedule II--Valuation and Qualifying Accounts--years ended December 31, 1999, 1998 and 1997.                   78

3.  Exhibits:

Refer to the exhibits listing beginning on the following page.

    (b) Reports on Form 8-k:

        None
</TABLE>
                                       3
<PAGE>

                               NSTAR (REGISTRANT)

<TABLE>
<CAPTION>
Exhibit 2   Plan of Acquisition, Reorganization, Arrangement, Liquidation or Seccession
----------  ----------------------------------------------------------------------------------------------------------
<C>         <S>
2.1         Amended and Restated Agreement and Plan of Merger, dated as of December 5, 1998, amended and restated as
            of May 4, 1999, by and among BEC Energy, Commonwealth Energy System, NSTAR, BEC Acquisition LLC and CES
            Acquisition LLC (Incorporated by reference to Annex A to the Joint Proxy Statement/Prospectus,
            Registration Statement on Form S-4 of NSTAR (No. 333-78285)).

Exhibit 3   Articles of Incorporation and By-Laws
----------  ----------------------------------------------------------------------------------------------------------
3.1         Declaration of Trust of NSTAR (incorporated by reference to Annex D to the Joint Proxy
            Statement/Prospectus, which forms part of the Registration Statement on Form S-4 of NSTAR (No.
            333-78285)).
3.2         Bylaws of NSTAR (Incorporated by reference to Annex E to the Joint Proxy Statement/Prospectus, which
            forms part of the Registration Statement on Form S-4 of NSTAR (No. 333-78285)).

Exhibit 4   Instruments Defining the Rights of Security Holders, Including Indentures
----------  ----------------------------------------------------------------------------------------------------------
4.0         Management agrees to furnish to the Securities and Exchange Commission, upon request, a copy of any other
            agreements or instruments of the Registrant and its subsidiaries defining the rights of holders of any
            long-term debt  whose authorization does not exceed 10% of total assets.
4.1         Indenture dated as of January 12, 2000 between NSTAR and Bank One Trust Company N.A. (Incorporated by
            reference, Exhibit 4.1 to NSTAR Registration Statement on Form S-3, File No. 333-94735).

Exhibit 10  Material Contracts
----------  ------------------------------------------------------------------------------------------------------
10.1        NSTAR Excess Benefit Plan, effective August 25, 1999 (Filed Herewith).
10.2        NSTAR Supplemental Executive Retirement Plan, effective August 25, 1999 (Filed Herewith).
10.3        Special Supplemental Executive Retirement Agreement between Boston Edison Company and Thomas J. May
            dated March 13, 1999, regarding Key Executive Benefit Plan and Supplemental Executive Retirement Plan
            (Filed Herewith).
10.4        Key Executive Benefit Plan Agreement dated as of October 1, 1983 between Boston Edison Company and
            Thomas J. May (Filed Herewith).
10.5        Key Executive Benefit Plan Agreement dated September 1, 1989 between Boston Edison Company and Ronald
            A. Ledgett (Filed Herewith).
10.6        Employment Agreement between Thomas J. May and NSTAR dated May 11, 1999 (Incorporated by reference to
            Annex A to the Joint Proxy Statement/Prospectus in Part I of the Registration Statement of NSTAR on
            Form S-4 (File No. 333-78285)).
10.7        Employment Agreement between Russell D. Wright and NSTAR dated May 11, 1999 (Incorporated by
            reference to Annex A to the Joint Proxy Statement/Prospectus in Part I of the Registration Statement
            of NSTAR on Form S-4 (File No. 333-78285)).
10.8        Employment Agreement between Boston Edison Company and Ronald A. Ledgett dated April 30, 1987 (Boston
            Edison Company Form 10-K for the year ended December 31, 1994, File No. 1-2301).
10.9        Change in Control Agreement between NSTAR and Thomas J. May dated May 11, 1999 (Filed Herewith).

10.10       Change in Control Agreement between NSTAR and Russell D. Wright dated May 11,1999 (Filed Herewith).
</TABLE>

                                       4
<PAGE>

<TABLE>
<S>         <C>
10.11       NSTAR Deferred Compensation Plan (Restated Effective August 25, 1999) (Filed Herewith).
10.12       NSTAR 1997 Share Incentive Plan, as amended June 30, 1999 and assumed by NSTAR effective August 28,
            2000 (Filed Herewith)

                          BEC Energy and Subsidiaries

Exhibit 3   Articles of Incorporation and By-Laws
----------  ------------------------------------------------------------------------------------------------------
3.1         Boston Edison Restated Articles of Organization (Form 10-Q for the quarter ended June 30, 1994, File
            No. 1-2301).
3.2         Boston Edison Company Bylaws April 19, 1977, as amended January 22, 1987, January 28, 1988, May 28,
            1988, and November 22, 1989 (Form 10-Q for the quarter ended June 30, 1990, File No. 1-2301).

Exhibit 4   Instruments Defining the Rights of Security Holders, Including Indentures
----------  ------------------------------------------------------------------------------------------------------
4.1         Medium-Term Notes Series A--Indenture dated September 1, 1988, between Boston Edison Company and Bank
            of Montreal Trust Company (Form 10-Q for the quarter ended September 30, 1988, File No. 1-2301).
4.1.1       First Supplemental Indenture dated June 1, 1990 to Indenture dated September 1, 1988 with Bank of
            Montreal Trust Company 9/7//8% debentures due June 1, 2020. (Form 8-K dated June 28, 1990, File No.
            1-2301).
4.1.26      Indenture of Trust and Agreement among the City of Boston, Massachusetts (acting by and through its
            Industrial Development Financing Authority) and Harbor Electric Energy Company and Shawmut Bank,
            N.A., as Trustee, dated November 1, 1991 (Form 10-K for the year end December 31, 1991, File No.
            1-2301).
4.1.27      Votes of the Pricing Committee of the Board of Directors of Boston Edison Company taken August 5,
            1991 re 9/3//8% debentures due August 15, 2021 (Form 10-K for the year ended December 31, 1991, File
            No. 1-2301)
4.1.25      Votes of the Pricing Committee of the Board of Directors of Boston Edison Company taken September 10,
            1992 re 8/1//4% debentures due September 15, 2022 (Form 10-K for the year ended December 31, 1997,
            File No. 1-2301).
4.1.26      Votes of the Pricing Committee of the Board of Directors of Boston Edison Company taken January 27,
            1993 re /6//80% debentures due February 1, 2000 (Form 10-K for the year ended December 31, 1992, File
            No. 1-2301).
4.1.27      Votes of the Pricing Committee of the Board of Directors of Boston Edison Company taken March 5, 1993
            re /6//80% Debentures due March 15, 2003 and 7.80% debentures due March 15, 2023 (Form 10-K for the
            year ended December 31, 1992, File No. 1-2301).
4.1.28      Votes of the Pricing Committee of the Board of Directors of Boston Edison Company taken August 18,
            1993 re 6.05% debentures due August 15, 2000 (Form 10-K for year ended December 31, 1993, File No.
            1-2301).
4.1.9       Votes of the Pricing Committee of the Board of Directors of Boston Edison Company taken May 10, 1995
            re 7.80% debentures due May 15, 2010 (Form 10-K for the year ended December 31, 1995, File No.
            1-2301).
</TABLE>

                                       5
<PAGE>

<TABLE>
<S>         <C>
Exhibit 10  Material Contracts
----------  ------------------------------------------------------------------------------------------------------
10.11       Boston Edison Company Deferred Fee Plan dated January 14, 1993 (Form 10-K for year ended December 31,
            1992, File No. 1-2301).
10.10       Deferred Compensation Trust between Boston Edison Company and State Street Bank and Trust Company
            dated February 2, 1993 (Form 10-K for the year ended December 31, 1992, File No. 1-2301).
10.5.1      Amendment No. 1 to Deferred Compensation Trust dated March 31, 1994 (Form 10-K for the year ended
            December 31, 1994).
10.10       Employment Agreement Applicable to Ronald A. Ledgett dated April 30, 1987 (Form 10-K for the year
            ended December 31, 1994, File No. 1-2301).
10.12       Boston Edison Company Restructuring Settlement Agreement dated July 1997 (Form 10-K for the year
            ended December 31, 1997, File No. 1-2301).
10.1        Boston Edison Company and Sithe Energies, Inc. Purchase and Sale and Transition Agreements dated
            December 10, 1997 (Form 10-Q for the quarter ended March 31, 1998, File No. 1-2301).
10.11       Boston Edison Company Directors' Deferred Fee Plan Restatement effective October 1, 1998 (Form 10-K
            for the year ended December 31, 1999, File No. 1-2301).
10.12       Boston Edison Company and Entergy Nuclear Generation Company Purchase and Sale Agreement dated
            November 18, 1998 (Form 10-K for the year ended December 31, 1999, File No. 1-2301).
10.13       License Agreement Between Boston Edison Company and Becocom, Inc., dated Jule 17, 1997 (Form 10-K for
            the year ended December 31, 1999, File No. 1-14768).
10.14       Chilled Water Service Agreement between Northwind Boston LLC and Prucenter Acquisition LLC, March 23,
            1999.  (Form 10-K for the year ended December 31, 1999, File No. 1-14768).

Exhibit 21  Subsidiaries of the Registrant
----------  ------------------------------------------------------------------------------------------------------
21.1        Boston Edison Company (incorporated in Massachusetts), a wholly owned subsidiary of BEC Energy.
21.2        Boston Energy Technology Group, Inc. (incorporated in Massachusetts), a wholly owned subsidiary of
            BEC Energy.

</TABLE>

                                       6
<PAGE>

<TABLE>

<S>         <C>
21.3        Harbor Electric Energy Company (incorporated in Massachusetts), a wholly owned subsidiary of Boston
            Edison Company.

Exhibit 99  Additional Exhibits
----------  ------------------------------------------------------------------------------------------------------
99.1        Settlement Agreement between Boston Edison Company and Commonwealth Electric Company, Montaup
            Electric Company and the Municipal Light Department of the Town of Reading, Massachusetts, dated
            January 5, 1990 (Form 8-K dated December 21, 1989, File No. 1-2301).
99.2        Settlement Agreement between Boston Edison Company and City of Holyoke Gas and City of Holyoke Gas
            and Electric Department et. al., dated April 26, 1990 (Form 10-Q for the quarter ended March 31,
            1990, File No. 1-2301).
99.3        Information required by SEC Form 11-K for certain employee benefit plans for the years ended December
            31, 1997, 1996 and 1995 (Form 10-K/A Amendments to Form 10-K for the years December 31, 1997, 1996
            and 1995 dated June 25, 1998, June 26, 1997 and June 27, 1996 respectively.

                                            COMMONWEALTH ENERGY SYSTEM

Exhibit 4   Instruments Defining the Rights of Security Holders, Including Indentures
----------  ------------------------------------------------------------------------------------------------------
4.1.1       CES Note Agreement ($40 Million Privately Placed Senior Notes) dated June 28, 1989 (Exhibit 1 to the
            CES Form 10-Q (September 1989), File No. 1-7316).

Exhibit 10  Power Contract
----------  ------------------------------------------------------------------------------------------------------
10.1.1      Power contracts between CEC (Unit 1) and NBGEL and CEL dated December 1, 1965 (Exhibit 13(a)(1-4) to
            the CEC Form S-1, File No. 2-30057).
10.1.2      Power contract between Yankee Atomic Electric Company (YAEC) and CEL dated June 30, 1959, as amended
            April 1, 1975 (Refiled as Exhibit 1 to the 1991 CEL Form 10-K, File No. 2-7909).
10.1.2.1    Second, Third and Fourth Amendments to 10.1.2 as amended October 1, 1980, April 1, 1985 and May 6,
            1988, respectively (Exhibit 2 to the CEL Form 10-Q (June 1988), File No. 2-7909).
10.1.2.2    Fifth and Sixth Amendments to 10.1.2 as amended June 26, 1989 and July 1, 1989, respectively (Exhibit
            1 to the CEL Form 10-Q (September 1989), File No. 2-7909).
10.1.3      Power Contract between YAEC and NBGEL dated June 30, 1959, as amended April 1, 1975 (Refiled as
            Exhibit 2 to the 1991 CE Form 10-K, File No. 2-7749).
10.1.3.1    Second, Third and Fourth Amendments to 10.1.3 as amended October 1, 1980, April 1, 1985 and May 6,
            1988, respectively (Exhibit 1 to the CE Form 10-Q (June 1988), File No. 2-7749).
10.1.3.2    Fifth and Sixth Amendments to 10.1.3 as amended June 26, 1989 and July 1, 1989, respectively (Exhibit
            3 to the CE Form 10-Q (September 1989), File No. 2-7749).
10.1.4      Power Contract between Connecticut Yankee Atomic Power Company (CYAPC) and CEL dated July 1, 1964
            (Exhibit 13-K1 to the Parent's Form S-1, (April 1967) File No. 2-25597).
10.1.4.1    Additional Power Contract providing for extension on contract term between CYAPC and CEL dated April
            30, 1984 (Exhibit 5 to the CEL Form 10-Q (June 1984), File No. 2-7909).
10.1.4.2    Second Supplementary Power Contract providing for decommissioning financing between CYAPC and CEL
            dated April 30, 1984 (Exhibit 6 to the CEL Form 10-Q (June 1984), File No. 2-7909).
10.1.5      Power contract between Vermont Yankee Nuclear Power Corporation (VYNPC) and CEL dated February 1,
            1968 (Exhibit 3 to the CEL 1984 Form 10-K, File No. 2-7909).

</TABLE>

                                       7
<PAGE>

<TABLE>

<S>        <C>
10.1.5.1    First Amendment dated June 1, 1972 (Section 7) and Second Amendment dated April 15, 1983
            (decommissioning financing) to 10.1.5 (Exhibits 1 and 2, respectively, to the CEL Form 10-Q (June
            1984), File No. 2-7909).
10.1.5.2    Third Amendment dated April 1, 1985 and Fourth Amendment dated June 1, 1985 to 10.1.5 (Exhibits 1 and
            2, respectively, to the CEL Form 10-Q (June 1986), File No. 2-7909).
10.1.5.3    Fifth and Sixth Amendments to 10.1.5 dated February 1, 1968, both as amended May 6, 1988 (Exhibit 1
            to the CEL Form 10-Q (June 1988), File No. 2-7909).
10.1.5.4    Seventh Amendment to 10.1.5 dated February 1, 1968, as amended June 15, 1989 (Exhibit 2 to the CEL
            Form 10-Q (September 1989), File No. 2-7909).
10.1.5.5    Additional Power Contract dated February 1, 1984 between CEL and VYNPC providing for decommissioning
            financing and contract extension (Refiled as Exhibit 1 to CEL 1993 Form 10-K, File No. 2-7909).
10.1.6      Power contract between Maine Yankee Atomic Power Company (MYAPC) and CEL dated May 20, 1968 (Exhibit
            5 to the Parent's Form S-7, File No. 2-38372).
10.1.6.1    First Amendment dated March 1, 1984 (decommissioning financing) and Second Amendment dated January 1,
            1984 (supplementary payments) to 10.1.6 (Exhibits 3 and 4 to the CEL Form 10-Q (June 1984), File No.
            2-7909).
10.1.6.2    Third Amendment to 10.1.6 dated October 1, 1984 (Exhibit 1 to the CEL Form 10-Q (September 1984),
            File No. 2-7909).
10.1.7      Agreement between NBGEL and Boston Edison Company (BECO) for the purchase of electricity from BECO's
            Pilgrim Unit No. 1 dated August 1, 1972 (Exhibit 7 to the CE 1984 Form 10-K, File No. 2-7749).
10.1.7.1    Service Agreement between NBGEL and BECO for purchase of stand-by power for BECO's Pilgrim Station
            dated August 16, 1978 (Exhibit 1 to the CE 1988 Form 10-K, File No. 2-7749).
10.1.7.2    System Power Sales Agreement by and between CE and BECO dated July 12, 1984 (Exhibit 1 to the CE Form
            10-Q (September 1984), File No. 2-7749).
10.1.7.3    Power Exchange Agreement by and between BECO and CE dated December 1, 1984 (Exhibit 16 to the CE 1984
            Form 10-K, File No. 2-7749).
10.1.7.4    Service Agreement for Non-Firm Transmission Service between BECO and CEL dated July 5, 1984 (Exhibit
            4 to the CEL 1984 Form 10-K, File No. 2-7909).
10.1.8      Agreement for Joint-Ownership, Construction and Operation of New Hampshire Nuclear Units (Seabrook)
            dated May 1, 1973 (Exhibit 13(N) to the NBGEL Form S-1 dated October 1973, File No. 2-49013 and as
            amended below:
10.1.8.1    First through Fifth Amendments to 10.1.8 as amended May 24, 1974, June 21, 1974, September 25, 1974,
            October 25, 1974 and January 31, 1975, respectively (Exhibit 13(m) to the NBGEL Form S-1 (November 7,
            1975), File No. 2-54995).
10.1.8.2    Sixth through Eleventh Amendments to 10.1.8 as amended April 18, 1979, April 25, 1979, June 8, 1979,
            October 11, 1979 and December 15, 1979, respectively (Refiled as Exhibit 1 to the CEC 1989 Form 10-K,
            File No. 2-30057).

</TABLE>

                                       8
<PAGE>

<TABLE>
<S>        <C>
10.1.8.3    Twelfth through Fourteenth Amendments to 10.1.8 as amended May 16, 1980, December 31, 1980 and June
            1, 1982, respectively (Filed as Exhibits 1, 2, and 3 to the CE 1992 Form 10-K, File No. 2-7749).
10.1.8.4    Fifteenth and Sixteenth Amendments to 10.1.8 as amended April 27, 1984 and June 15, 1984,
            respectively (Exhibit 1 to the CEC Form 10-Q (June 1984), File No. 2-30057).
10.1.8.5    Seventeenth Amendment to 10.1.8 as amended March 8, 1985 (Exhibit 1 to the CEC Form 10-Q (March
            1985), File No. 2-30057).
10.1.8.6    Eighteenth Amendment to 10.1.8 as amended March 14, 1986 (Exhibit 1 to the CEC Form 10-Q (March
            1986), File No. 2-30057).
10.1.8.7    Nineteenth Amendment to 10.1.8 as amended May 1, 1986 (Exhibit 1 to the CEC Form 10-Q (June 1986),
            File No. 2-30057).
10.1.8.8    Twentieth Amendment to 10.1.8 as amended September 19, 1986 (Exhibit 1 to the CEC 1986 Form 10-K,
            File No. 2-30057).
10.1.8.9    Twenty-First Amendment to 10.1.8 as amended November 12, 1987 (Exhibit 1 to the CEC 1987 Form 10-K,
            File No. 2-30057).
10.1.8.10   Settlement Agreement and Twenty-Second Amendment to 10.1.8, both dated January 13, 1989 (Exhibit 4 to
            the CEC 1988 Form 10-K, File No. 2-30057).
10.1.9      Purchase and Sale Agreement together with an implementing Addendum dated December 31, 1981, between
            CE and CEC, for the purchase and sale of the CE 3.52% joint-ownership interest in the Seabrook units,
            dated January 2, 1981 (Refiled as Exhibit 4 to the CE 1992 Form 10-K, File No. 2-7749).
10.1.10     Agreement to transfer ownership, construction and operational interest in the Seabrook Units 1 and 2
            from CE to CEC dated January 2, 1981 (Refiled as Exhibit 3 to the 1991 CE Form 10-K, File No. 2-7749).
10.1.11     Power Contract, as amended to February 28, 1990, superseding the Power Contract dated September 1,
            1986 and amendment dated June 1, 1988, between CEC (seller) and CE and CEL (purchasers) for seller's
            entire share of the Net Unit Capability of Seabrook 1 and related energy (Exhibit 1 to the CEC Form
            10-Q (March 1990), File No. 2-30057).
10.1.12     Agreement between NBGEL and Central Maine Power Company (CMP), for the joint-ownership, construction
            and operation of William F. Wyman Unit No. 4 dated November 1, 1974 together with Amendment No. 1
            dated June 30, 1975 (Exhibit 13(N) to the NBGEL Form S-1, File No. 2-54955).
10.1.12.1   Amendments No. 2 and 3 to 10.1.12 as amended August 16, 1976 and December 31, 1978 (Exhibit 5(a) 14
            to the Parent's Form S-16 (June 1979), File No. 2-64731).
10.1.13     Agreement between the registrant and Montaup Electric Company (MEC) for use of common facilities at
            Canal Units I and II and for allocation of related costs, executed October 14, 1975 (Exhibit 1 to the
            CEC 1985 Form 10-K, File No. 2-30057).
10.1.13.1   Agreement between the registrant and MEC for joint-ownership of Canal Unit II, executed October 14,
            1975 (Exhibit 2 to the CEC 1985 Form 10-K, File No. 2-30057).
10.1.13.2   Agreement between the registrant and MEC for lease relating to Canal Unit II, executed October 14,
            1975 (Exhibit 3 to the CEC 1985 Form 10-K, File No. 2-30057).
10.1.14     Contract between CEC and NBGEL and CEL, affiliated companies, for the sale of specified amounts of
            electricity from Canal Unit 2 dated January 12, 1976 (Exhibit 7 to the Parent's 1985 Form 10-K, File
            No. 1-7316).

</TABLE>

                                       9
<PAGE>

<TABLE>
<S>        <C>
10.1.15     Capacity Acquisition Agreement between CEC, CEL and CE dated September 25, 1980 (Refiled as Exhibit 1
            to the 1991 CEC Form 10-K, File No. 2-30057).
10.1.15.1   Amendment to 10.1.15 as amended and restated June 1, 1993, henceforth referred to as the Capacity
            Acquisition and Disposition Agreement, whereby Canal Electric Company, as agent, in addition to
            acquiring power may also sell bulk electric power which Cambridge Electric Light Company and/or
            Commonwealth Electric Company owns or otherwise has the right to sell (Exhibit 1 to Canal Electric's
            Form 10-Q (September 1993), File No. 2-30057).
10.1.16     Phase 1 Vermont Transmission Line Support Agreement and Amendment No. 1 thereto between Vermont
            Electric Transmission Company, Inc. and certain other New England utilities, dated December 1, 1981
            and June 1, 1982, respectively (Exhibits 5 and 6 to the CE 1992 Form 10-K, File No. 2-7749).
10.1.16.1   Amendment No. 2 to 10.1.16 as amended November 1, 1982 (Exhibit 5 to the CE Form 10-Q (June 1984),
            File No. 2-7749).
10.1.16.2   Amendment No. 3 to 10.1.16 as amended January 1, 1986 (Exhibit 2 to the CE 1986 Form 10-K, File No.
            2-7749).
10.1.17     Power Purchase Agreement between Pioneer Hydropower, Inc. and CE for the purchase of available
            hydro-electric energy produced by a facility located in Ware, Massachusetts, dated September 1, 1983
            (Refiled as Exhibit 1 to the CE 1993 Form 10-K, File No. 2-7749).
10.1.18     Power Purchase Agreement between Corporation Investments, Inc. (CI), and CE for the purchase of
            available hydro-electric energy produced by a facility located in Lowell, Massachusetts, dated
            January 10, 1983 (Refiled as Exhibit 2 to the CE 1993 Form 10-K, File No. 2-7749).
10.1.18.1   Amendment to 10.1.18 between CI and Boott Hydropower, Inc., an assignee therefrom, and CE, as amended
            March 6, 1985 (Exhibit 8 to the CE 1984 Form 10-K, File No. 2-7749).
10.1.19     Phase 1 Terminal Facility Support Agreement dated December 1, 1981, Amendment No. 1 dated June 1,
            1982 and Amendment No. 2 dated November 1, 1982, between New England Electric Transmission
            Corporation (NEET), other New England utilities and CE (Exhibit 1 to the CE Form 10-Q (June 1984),
            File No. 2-7749).
10.1.19.1   Amendment No. 3 to 10.1.19 (Exhibit 2 to the CE Form 10-Q (June 1986), File No. 2-7749).
10.1.20     Preliminary Quebec Interconnection Support Agreement dated May 1, 1981, Amendment No. 1 dated
            September 1, 1981, Amendment No. 2 dated June 1, 1982, Amendment No. 3 dated November 1, 1982,
            Amendment No. 4 dated March 1, 1983 and Amendment No. 5 dated June 1, 1983 among certain New England
            Power Pool (NEPOOL) utilities (Exhibit 2 to the CE Form 10-Q (June 1984), File No. 2-7749).
10.1.21     Agreement with Respect to Use of Quebec Interconnection dated December 1, 1981, Amendment No. 1 dated
            May 1, 1982 and Amendment No. 2 dated November 1, 1982 among certain NEPOOL utilities (Exhibit 3 to
            the CE Form 10-Q (June 1984), File No. 2-7749).
10.1.21.1   Amendatory Agreement No. 3 to 10.1.21 as amended June 1, 1990, among certain NEPOOL utilities
            (Exhibit 1 to the CEC Form 10-Q (September 1990), File No. 2-30057).
10.1.22     Phase I New Hampshire Transmission Line Support Agreement between NEET and certain other New England
            Utilities dated December 1, 1981 (Exhibit 4 to the CE Form 10-Q (June 1984), File No. 2-7749).
10.1.23     Agreement, dated September 1, 1985, with Respect To Amendment of Agreement With Respect To Use Of
            Quebec Interconnection, dated December 1, 1981, among certain NEPOOL utilities to include Phase II
            facilities in the definition of ''Project'' (Exhibit 1 to the CEC Form 10-Q (September 1985), File
            No. 2-30057).

</TABLE>

                                       10
<PAGE>

<TABLE>
<S>         <C>

10.1.24     Agreement to Preliminary Quebec Interconnection Support Agreement--Phase II among Public Service
            Company of New Hampshire (PSNH), New England Power Co. (NEP), BECO and CEC whereby PSNH assigns a
            portion of its interests under the original Agreement to the other three parties, dated October 1,
            1987 (Exhibit 2 to the CEC 1987 Form 10-K, File No. 2-30057).
10.1.25     Preliminary Quebec Interconnection Support Agreement--Phase II among certain New England electric
            utilities dated June 1, 1984 (Exhibit 6 to the CE Form 10-Q (June 1984), File No. 2-7749).
10.1.25.1   First, Second and Third Amendments to 10.1.25 as amended March 1, 1985, January 1, 1986 and March 1,
            1987, respectively (Exhibit 1 to the CEC Form 10-Q (March 1987), File No. 2-30057).
10.1.25.2   Fifth, Sixth and Seventh Amendments to 10.1.25 as amended October 15, 1987, December 15, 1987 and
            March 1, 1988, respectively (Exhibit 1 to the CEC Form 10-Q (June 1988), File No. 2-30057).
10.1.25.3   Fourth and Eighth Amendments to 10.1.25 as amended July 1, 1987 and August 1, 1988, respectively
            (Exhibit 3 to the CEC Form 10-Q (September 1988), File No. 2-30057).
10.1.25.4   Ninth and Tenth Amendments to 10.1.25 as amended November 1, 1988 and January 15, 1989, respectively
            (Exhibit 2 to the CEC 1988 Form 10-K, File No. 2-30057).
10.1.25.5   Eleventh Amendment to 10.1.25 as amended November 1, 1989 (Exhibit 4 to the CEC 1989 Form 10-K, File
            No. 2-30057).
10.1.25.6   Twelfth Amendment to 10.1.25 as amended April 1, 1990 (Exhibit 1 to the CEC Form 10-Q (June 1990),
            File No. 2-30057).
10.1.26     Phase II Equity Funding Agreement for New England Hydro-Transmission Electric Company, Inc. (New
            England Hydro) (Massachusetts), dated June 1, 1985, between New England Hydro and certain NEPOOL
            utilities (Exhibit 2 to the CEC Form 10-Q (September 1985), File No. 2-30057).
10.1.27     Phase II Massachusetts Transmission Facilities Support Agreement dated June 1, 1985, refiled as a
            single agreement incorporating Amendments 1 through 7 dated May 1, 1986 through January 1, 1989,
            respectively, between New England Hydro and certain NEPOOL utilities (Exhibit 2 to the CEC Form 10-Q
            (September 1990), File No. 2-30057).
10.1.28     Phase II New Hampshire Transmission Facilities Support Agreement dated June 1, 1985, refiled as a
            single agreement incorporating Amendments 1 through 8 dated May 1, 1986 through January 1, 1990,
            respectively, between New England Hydro-Transmission Corporation (New Hampshire Hydro) and certain
            NEPOOL utilities (Exhibit 3 to the CEC Form 10-Q (September 1990), File No. 2-30057).
10.1.29     Phase II Equity Funding Agreement for New Hampshire Hydro, dated June 1, 1985, between New Hampshire
            Hydro and certain NEPOOL utilities (Exhibit 3 to the CEC Form 10-Q (September 1985), File No.
            2-30057).
10.1.29.1   Amendment No. 1 to 10.1.29 dated May 1, 1986 (Exhibit 6 to the CEC Form 10-Q (March 1987), File No.
            2-30057).
10.1.29.2   Amendment No. 2 to 10.1.29 as amended September 1, 1987 (Exhibit 3 to the CEC Form 10-Q (September
            1987), File No. 2-30057).
10.1.30     Phase II New England Power AC Facilities Support Agreement, dated June 1, 1985, between NEP and
            certain NEPOOL utilities (Exhibit 6 to the CEC Form 10-Q (September 1985), File No. 2-30057).
10.1.30.1   Amendments Nos. 1 and 2 to 10.1.30 as amended May 1, 1986 and February 1, 1987, respectively (Exhibit
            5 to the CEC Form 10-Q (March 1987), File No. 2-30057).

</TABLE>

                                       11
<PAGE>

<TABLE>
<S>         <C>


10.1.30.2   Amendments Nos. 3 and 4 to 10.1.30 as amended June 1, 1987 and September 1, 1987, respectively
            (Exhibit 5 to the CEC Form 10-Q (September 1987), File No. 2-30057).
10.1.31     Agreement Authorizing Execution of Phase II Firm Energy Contract, dated September 1, 1985, among
            certain NEPOOL utilities in regard to participation in the purchase of power from Hydro-Quebec
            (Exhibit 8 to the CEC Form 10-Q (September 1985), File No. 2-30057).
10.1.32     Agreements by and between Swift River Company and CE for the purchase of available hydro-electric
            energy to be produced by units located in Chicopee and North Willbraham, Massachusetts, both dated
            September 1, 1983 (Exhibits 11 and 12 to the CE 1984 Form 10-K, File No. 2-7749).
10.1.33     Power Purchase Agreement by and between SEMASS Partnership, as seller, to construct, operate and own
            a solid waste disposal facility at its site in Rochester, Massachusetts and CE, as buyer of electric
            energy and capacity, dated September 8, 1981 (Exhibit 17 to the CE 1984 Form 10-K, File No. 2-7749).
10.1.33.1   Power Sales Agreement to 10.1.33 for all capacity and related energy produced, dated October 31, 1985
            (Exhibit 2 to the CE 1985 Form 10-K, File No. 2-7749).
10.1.33.2   Amendment to 10.1.33 for all additional electric capacity and related energy to be produced by an
            addition to the Original Unit, dated March 14, 1990 (Exhibit 1 to the CE Form 10-Q (June 1990), File
            No. 2-7749).
10.1.33.3   Amendment to 10.1.33 for all additional electric capacity and related energy to be produced by an
            addition to the Original Unit, dated May 24, 1991 (Exhibit 1 to CE Form 10-Q (June 1991), File No.
            2-7749).
10.1.34     Power Sale Agreement by and between CE (buyer) and Northeast Energy Associated, Ltd. (NEA) (seller)
            of electric energy and capacity, dated November 26, 1986 (Exhibit 1 to the CE Form 10-Q (March 1987),
            File No. 2-7749).
</TABLE>

                                       12
<PAGE>

<TABLE>
<C>         <S>
 10.1.34.1  First Amendment to 10.1.34 as amended August 15, 1988 (Exhibit 1 to the CE Form 10-Q (September
            1988), File No. 2-7749).
 10.1.34.2  Second Amendment to 10.1.34 as amended January 1, 1989 (Exhibit 2 to the CE 1988 Form 10-K, File No.
            2-7749).
 10.1.34.3  Power Sale Agreement dated August 15, 1988 between NEA and CE for the purchase of 21 MW of
            electricity (Exhibit 2 to the CE Form 10-Q (September 1988), File No. 2-7749).
 10.1.34.4  Amendment to 10.1.34.3 as amended January 1, 1989 (Exhibit 3 to the CE 1988 Form 10-K, File No.
            2-7749).
 10.1.35    Power Purchase Agreement and First Amendment, dated September 5, 1989 and August 3, 1990,
            respectively, by and between Commonwealth Electric (buyer) and Dartmouth Power Associates Limited
            Partnership (seller), whereby buyer will purchase all of the energy (67.6 MW) produced by a single
            gas turbine unit (Exhibit 1 to the CE Form 10-Q (June 1992), File No. 2-7749).
 10.1.35.1  Second Amendment, dated June 23, 1994, to 10.1.50 by and between Commonwealth Electric Company and
            Dartmouth Power Associates, L.P. dated September 5, 1989 (Exhibit 4 to the CE Form 10-Q (June 1995),
            File No. 2-7749).
 10.1.36    Power Purchase Agreement by and between Masspower (seller) and Commonwealth Electric Company (buyer)
            for a 11.11% entitlement to the electric capacity and related energy of a 240 MW gas-fired
            cogeneration facility, dated February 14, 1992 (Exhibit 1 to Commonwealth Electric's Form 10-Q
            (September 1993), File No. 2-7749).
 10.1.37    Power Sale Agreement by and between Altresco Pittsfield, L.P. (seller) and Commonwealth Electric
            Company (buyer) for a 17.2% entitlement to the electric capacity and related energy of a 160 MW
            gas-fired cogeneration facility, dated February 20, 1992 (Exhibit 2 to Commonwealth Electric's Form
            10-Q (September 1993), File No. 2-7749).
 10.1.37.1  System Exchange Agreement by and among Altresco Pittsfield, L.P., Cambridge Electric Light Company,
            Commonwealth Electric Company and New England Power Company, dated July 2, 1993 (Exhibit 3 to
            Commonwealth Electric's Form 10-Q (September 1993), File No 2-7749).
 10.1.37.2  Power Sale Agreement by and between Altresco Pittsfield, L. P. (seller) and Cambridge Electric Light
            Company (Cambridge Electric) (buyer) for a 17.2% entitlement to the electric capacity and related
            energy of a 160 MW gas-fired cogeneration facility, dated February 20, 1992 (Exhibit 1 to Cambridge
            Electric's Form 10-Q (September 1993), File No. 2-7909).
 10.1.37.3  First Amendment, dated November 7, 1994, to 10.1.37 by and between Commonwealth Electric Company and
            Altresco Pittsfield, L.P. dated February 20, 1992 (Filed as Exhibit 3 to Commonwealth Electric
            Company's Form 10-Q (June 1995), File 2-7749).
 10.1.37.4  First Amendment, dated November 7, 1994, to 10.1.37.2 by and between Cambridge Electric Light Company
            and Altresco Pittsfield, L.P. dated February 20, 1992 (Filed as Exhibit 2 to Cambridge Electric Light
            Company's Form 10-Q (June 1995), File 2-7909).
 10.2.1     Transportation Agreement between CNG and CG to provide for transportation of natural gas on a daily
            basis from Steuben Gas Storage Company to TGP (Exhibit 10 to the CG 1991 Form 10-K, File No. 2-1647).
 10.3.1     Pension Plan for Employees of Commonwealth Energy System and Subsidiary Companies as amended and
            restated January 1, 1993 (Exhibit 1 to CES Form 10-Q (September 1993), File No. 1-7316).
 10.3.2     Employees Savings Plan of Commonwealth Energy System and Subsidiary Companies as amended and restated
            January 1, 1993 (Exhibit 2 to CES Form 10-Q (September 1993), File No. 1-7316).

</TABLE>

                                       13
<PAGE>

<TABLE>
<S>         <C>

 10.3.2.1   First Amendment to 10.3.2, effective October 1, 1994. (Exhibit 1 to CES Form S-8 (January 1995), File
            No. 1-7316).
 10.3.2.2   Second Amendment to 10.3.2, effective April 1, 1996 (Exhibit 1 to CES Form 10-K/A Amendment No. 1
            (April 30, 1996), File No. 1-7316).
 10.3.2.3   Third Amendment to 10.3.2, effective January 1, 1997 (Exhibit 1 to CES Form 10-K/A Amendment No. 1
            (April 29, 1997), File No. 1-7316).
 10.3.3     New England Power Pool Agreement (NEPOOL) dated September 1, 1971 as amended through August 1, 1977,
            between NEGEA Service Corporation, as agent for CEL, CEC, NBGEL, and various other electric utilities
            operating in New England together with amendments dated August 15, 1978, January 31, 1979 and
            February 1, 1980. (Exhibit 5(c)13 to New England Gas and Electric Association's Form S-16 (April
            1980), File No. 2-64731).
 10.3.3.1   Thirteenth Amendment to 10.3.3 as amended September 1, 1981 (Refiled as Exhibit 3 to the Parent's
            1991 Form 10-K, File No. 1-7316).
 10.3.3.2   Fourteenth through Twentieth Amendments to 10.3.3 as amended December 1, 1981, June 1, 1982, June 15,
            1983, October 1, 1983, August 1, 1985, August 15, 1985 and September 1, 1985, respectively (Exhibit 4
            to the CES Form 10-Q (September 1985), File No. 1-7316).
 10.3.3.3   Twenty-first Amendment to 10.3.3 as amended to January 1, 1986 (Exhibit 1 to the CES Form 10-Q (March
            1986), File No. 1-7316).
 10.3.3.4   Twenty-second Amendment to 10.3.3 as amended to September 1, 1986 (Exhibit 1 to the CES Form 10-Q
            (September 1986), File No. 1-7316).
 10.3.3.5   Twenty-third Amendment to 10.3.3 as amended to April 30, 1987 (Exhibit 1 to the CES Form 10-Q (June
            1987), File No. 1-7316).
 10.3.3.6   Twenty-fourth Amendment to 10.3.3 as amended March 1, 1988 (Exhibit 1 to the CES Form 10-Q (March
            1989), File No. 1-7316).
 10.3.3.7   Twenty-fifth Amendment to 10.3.3. as amended to May 1, 1988 (Exhibit 1 to the CES Form 10-Q (March
            1988), File No. 1-7316).
 10.3.3.8   Twenty-sixth Agreement to 10.3.3 as amended March 15, 1989 (Exhibit 1 to the CES Form 10-Q (March
            1989), File No. 1-7316).
 10.3.3.9   Twenty-seventh Agreement to 10.3.3 as amended October 1, 1990 (Exhibit 3 to the CES 1990 Form 10-K,
            File No. 1-7316).
 10.3.3.10  Twenty-eighth Agreement to 10.3.3 as amended September 15, 1992 (Exhibit 1 to the CES Form 10-Q
            (September 1994), File No. 1-7316).
</TABLE>

                                       14
<PAGE>

<TABLE>
<C>         <S>
 10.3.3.11  Twenty-ninth Agreement to 10.3.3 as amended May 1, 1993 (Exhibit 2 to the CES Form 10-Q (September
            1994), File No. 1-7316).
 10.3.4     Guarantee Agreement by CEL (as guarantor) and MYA Fuel Company (as initial lender) covering the
            unconditional guarantee of a portion of the payment obligations of Maine Yankee Atomic Power Company
            under a loan agreement and note initially between Maine Yankee and MYA Fuel Company (Exhibit 3 to the
            CEL Form 10-K for 1985, File No. 2-7909).

                       CAMBRIDGE ELECTRIC LIGHT COMPANY

Exhibit 4   Instruments Defining the Rights of Security Holders, Including Indentures
----------  ------------------------------------------------------------------------------------------------------
 4.2.1      Original Indenture on Form S-1 (April, 1949) (Exhibit 7(a), File No. 2-7909).
 4.2.2      Third Supplemental on Form 10-K (1984) (Exhibit 1, File No. 2-7909).
 4.2.3      Fourth Supplemental on Form 10-K (1984) (Exhibit 2, File No. 2-7909).
 4.2.4      Sixth Supplemental on Form 10-Q (June 1989) (Exhibit 1, File No. 2-7909).
 4.2.5      Seventh Supplemental on Form 10-Q (June 1992), (Exhibit 1, File No. 2-7909).

                           COMMONWEALTH GAS COMPANY

Exhibit 4   Instruments Defining the Rights of Security Holders, Including Indentures
----------  ------------------------------------------------------------------------------------------------------
 4.4.1      Original Indenture on Form S-1 (Feb., 1949) (Exhibit 7(a), File No. 2-7820).
 4.4.2      Sixteenth Supplemental on Form 10-K (1986) (Exhibit 1, File No. 2-1647).
 4.4.3      Seventeenth Supplemental on Form 10-K (1990) (Exhibit 2, File No. 2-1647).
 4.4.4      Eighteenth Supplemental on Form 10-Q (March 1994) (Exhibit 1, File No. 2-1647).
 4.4.5      Nineteenth Supplemental on Form 10-K (1997) (Exhibit 1, File No. 2-1647).
</TABLE>

                                       15
<PAGE>

                                                                     SCHEDULE II

                       VALUATION AND QUALIFYING ACCOUNTS

               FOR THE YEARS ENDED DECEMBER 31, 1999, 1998, 1997

                             (DOLLARS IN THOUSANDS)

<TABLE>
<CAPTION>
                                                                   ADDITIONS
                                                            -----------------------
                                             BALANCE AT     PROVISIONS                 DEDUCTIONS     BALANCE
                                             BEGINNING      CHARGED TO                  ACCOUNTS      AT END
DESCRIPTION                                   OF YEAR       OPERATIONS   RECOVERIES   WRITTEN OFF     OF YEAR
----------------------------------------  ----------------  -----------  ----------   ------------  -----------
<S>                                       <C>               <C>          <C>          <C>           <C>
YEAR ENDED DECEMBER 31, 1999
Allowance for Doubtful Accounts......           $14,158(a)      $23,098       $5,260       $20,089      $22,427
YEAR ENDED DECEMBER 31, 1998
Allowance for Doubtful Accounts......           $10,228         $ 9,555       $4,242       $14,959      $ 9,066
YEAR ENDED DECEMBER 31, 1997
Allowance for Doubtful Accounts......           $ 2,000         $24,884       $3,593       $20,249      $10,228
</TABLE>

(a) The beginning balance includes $5,092,000 that relates to COM/Energy's
    reserve balance at the merger date of August 25, 1999.






                      CONSENT OF INDEPENDENT ACCOUNTANTS
                      ----------------------------------

We hereby consent to the incorporation by reference in the Registration
Statement on Form S-3 (No. 333-94735) of NStar of our report dated January 26,
2000 relating to the financial statements, which appears in the Annual Report to
Shareholders, which is incorporated in this Annual Report on Form 10-K. We also
consent to the incorporation by reference of our report dated January 26
relating to the financial statement schedules, which appears in this Form 10-K.


PricewaterhouseCoopers LLP
Boston, Massachusetts


                                       16
<PAGE>

FORM 10-K/A                         NSTAR                     DECEMBER 31, 1999
AMENDMENT NO. 1




                                   SIGNATURE


  Pursuant to the requirements of Section 13 or 15(d) of the Securities Exchange
Act of 1934, the registrant has duly caused this report to be signed on its
behalf by the undersigned, thereunto duly authorized.


                                             NSTAR
                                           ----------
                                          Registrant


                                        By: /s/   Robert J. Weafer, Jr.
                                            -------------------------------
                                                  ROBERT J. WEAFER, JR.
                                              VICE PRESIDENT, CONTROLLER AND
                                                 CHIEF ACCOUTING OFFICER

Date: September 29, 2000

                                       17