SECURITIES AND EXCHANGE COMMISSION
WASHINGTON, D.C. 20549

SCHEDULE 13G

UNDER THE SECURITIES EXCHANGE ACT OF 1934
(AMENDMENT NO. )*

NRG ENERGY, INC.
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(Name of Issuer)

Common Stock
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(Title of Class of Securities)

62937710
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(CUSIP Number)

Check the following box if a fee is being paid with this |_|  statement.  (A fee
is not required only if the filing person:  (1) has a previous statement on file
reporting  beneficial  ownership  of more  than  five  percent  of the  class of
securities  described  in Item 1;  and (2) has  filed  no  amendment  subsequent
thereto reporting  beneficial  ownership of five percent or less of such class.)
(See Rule 13d-7.)

*The  remainder of this cover page shall be filled out for a reporting  person's
initial filing on this form with respect to the subject class of securities, and
for any  subsequent  amendment  containing  information  which  would  alter the
disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not be deemed
to be "filed" for the purpose of Section 18 of the  Securities  Exchange  Act of
1934 ("Act") or otherwise  subject to the liabilities of that section of the Act
but  shall be  subject  to all other  provisions  of the Act  (however,  see the
Notes).

(Continued on following page(s))

Page 1 of 5 Pages

SEC 1745 (10-85)

CUSIP NO. 62937710                     13G             PAGE 2 OF 5 PAGES

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1    NAME OF REPORTING PERSON
S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON

Massachusetts Financial Services Company ("MFS")
I.R.S. Identification No.:  04-2747644
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2    CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*

(a)   /   /           (b)     /   /
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3    SEC USE ONLY
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4    CITIZENSHIP OR PLACE OF ORGANIZATION

Delaware
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NUMBER OF        5    SOLE VOTING POWER

SHARES            3,526,870 shares of common stock

BENEFICIALLY
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OWNED BY        6    SHARED VOTING POWER

EACH
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REPORTING        7    SOLE DISPOSITIVE POWER

PERSON             3,645,780 shares of common stock

WITH
--------------------------------------------------------------------------------
8    SHARED DISPOSITIVE POWER

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9    AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

3,645,780 shares of common stock of which 1,653,010 shares are also
beneficially owned by MFS Series Trust VII - MFS Capital Opportunities
Fund (see page 3) and 1,992,770 shares are also owned by certain other
non-reporting entities.
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10   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES*

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11   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9

11.3%
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12   TYPE OF REPORTING PERSON*

IA
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*SEE INSTRUCTION BEFORE FILLING OUT!

CUSIP NO. 62937710                  13G                   PAGE 3 OF 5 PAGES

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1    NAME OF REPORTING PERSON
S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON

MFS Series Trust VII - MFS Capital Opportunities Fund ("MVF")
I.R.S. Identification No.:  04-6490290
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2    CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*

(a)   /   /           (b)     /   /
--------------------------------------------------------------------------------
3    SEC USE ONLY
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4    CITIZENSHIP OR PLACE OF ORGANIZATION

Massachusetts
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NUMBER OF        5    SOLE VOTING POWER

SHARES

BENEFICIALLY
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OWNED BY        6    SHARED VOTING POWER

EACH
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REPORTING        7    SOLE DISPOSITIVE POWER

PERSON

WITH
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8    SHARED DISPOSITIVE POWER

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9    AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

1,653,010 shares of common stock (as noted on page 2, Item 9, MFS is also
a beneficial owner of these 1,653,010 shares).
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10   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES*

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11   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9

5.1%
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12   TYPE OF REPORTING PERSON*

IV
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*SEE INSTRUCTION BEFORE FILLING OUT!

SCHEDULE 13G                                           PAGE 4 OF 5 PAGES

ITEM 1:   (a)   NAME OF ISSUER:

NRG Energy, Inc.

(b)   ADDRESS OF ISSUER'S PRINCIPAL EXECUTIVE OFFICES:

1221 Nicollet Mall
Suite 700
Minneapolis, MN  55403-2445

ITEM 2:   (a)   NAME OF PERSON FILING:

Massachusetts Financial Services Company ("MFS")*

(b)   ADDRESS OF PRINCIPAL BUSINESS OFFICE OR, IF NONE, RESIDENCE:

For MFS and MVF:
500 Boylston Street
Boston, MA  02116

(c)   CITIZENSHIP:

For MFS, see Item 4 on page 2; for MVF, see Item 4 on page 3

(d)   TITLE OF CLASS OF SECURITIES:

Common Stock

(e)   CUSIP NUMBER:

62937710

ITEM 3:         For MFS, see Item 12 on page 2; for MVF, see Item 12 on page 3

ITEM 4:   (a)   AMOUNT BENEFICIALLY OWNED:

For MFS, see Item 9 on page 2; for MVF, see Item 9 on page 3

(b)   PERCENT OF CLASS:

For MFS, see Item 11 on page 2; for MVF, see Item 11 on page 3

(c)   NUMBER OF SHARES AS  TO  WHICH  SUCH  PERSON  HAS  VOTING  AND
DISPOSITIVE POWERS:

For MFS, see Items 5 and 7 on page 2
--------------------------
* This Schedule 13G is also filed  pursuant to Rule 13d-1(f) on behalf of MFS
Series Trust VII - MFS Capital Opportunities Fund ("MVF")(see page 3 and 
Exhibit 1 attached hereto).

SCHEDULE 13G                                           PAGE 5 OF 5 PAGES

ITEM 5:         OWNERSHIP OF FIVE PERCENT OR LESS OF A CLASS:

Inapplicable

ITEM 6:         OWNERSHIP OF MORE THAN FIVE PERCENT ON BEHALF OF ANOTHER PERSON:

Inapplicable

ITEM 7:         IDENTIFICATION AND CLASSIFICATION OF THE SUBSIDIARY WHICH 
ACQUIRED  THE  SECURITY  BEING  REPORTED  ON BY  THE  PARENT
HOLDING COMPANY:

Inapplicable

ITEM 8:         IDENTIFICATION AND CLASSIFICATION OF MEMBERS OF THE GROUP:

Inapplicable

ITEM 9:         NOTICE OF DISSOLUTION OF GROUP:

Inapplicable

ITEM 10:        CERTIFICATION:

By signing  below I certify that to the best of my knowledge and
belief, the securities referred to above were acquired in the ordinary course of
business  and were not acquired for the purpose of and do not have the effect of
changing or  influencing  the control of the issuer of such  securities and were
not acquired in connection  with or as a participant in any  transaction  having
such purposes or effect.

After  reasonable  inquiry and to the best of my  knowledge  and
belief,  I certify  that the  information  set forth in this  statement is true,
complete and correct.

Date:  July 14, 2000

Massachusetts Financial Services Company

By:       STEPHEN E. CAVAN
Stephen E. Cavan
Senior Vice President,
Secretary and General Counsel

EXHIBIT 1

MFS(R) CAPITAL OPPORTUNITIES FUND, a series of
MFS(R) SERIES TRUST VII
500 BOYLSTON STREET o BOSTON o MASSACHUSETTS 02116
617 o 954-5000

January 26, 1999

Massachusetts Financial Services Company
500 Boylston Street
Boston, MA  02116

Re:   Rule 13d-1

Gentlemen:

This letter is to  memorialize  our  agreement  that you shall file all
statements  on  Schedule  13G  required  to be filed on  behalf  of MFS  Capital
Opportunities  Fund,  a series of MFS Series  Trust VII,  pursuant to Rule 13d-1
under the  Securities  Exchange Act of 1934.  If the  foregoing is acceptable to
you, please sign and return to us the enclosed copy of this letter.

Very truly yours,

W. THOMAS LONDON
W. Thomas London
Treasurer

The foregoing is acceptable.

MASSACHUSETTS FINANCIAL SERVICES COMPANY

By:    ARNOLD D. SCOTT
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Arnold D. Scott
Senior Executive Vice President,
Secretary and Director