<PAGE>

                                                               EXHIBIT 4.1(a)


                                 AMENDMENT NO. 1

     AMENDMENT NO. 1 dated as of September 17, 2001, between METROMEDIA FIBER
NETWORK, INC., a Delaware corporation (the "ISSUER"), each of the Guarantors
listed on the signature pages hereto under the caption "GUARANTORS" (each a
"GUARANTOR" and, collectively, the "GUARANTORS", and the Guarantors collectively
with the Issuer, the "OBLIGORS"), each of the purchasers whose names appear on
the signature pages hereto under the caption "PURCHASERS" (each a "PURCHASER"
and, collectively, the "PURCHASERS") and Citicorp USA, Inc., as administrative
agent (in such capacity, together with its successors in such capacity, the
"ADMINISTRATIVE AGENT").

     The Issuer, the Guarantors, the Purchasers and the Administrative Agent are
parties to a Note and Guarantee Agreement dated as of September 6, 2001
providing, subject to the terms and conditions thereof, for the purchase of
notes in an aggregate principal amount equal to $150,000,000. The parties hereto
wish to amend the Note and Guarantee Agreement in certain respects and,
accordingly, hereby agree as follows:

     Section 1. DEFINITIONS. Except as otherwise defined in this Amendment No.
1, terms defined in the Note and Guarantee Agreement are used herein as defined
therein.

     Section 2. AMENDMENTS. Subject to the execution and delivery hereof by each
of the parties hereto, the Note and Guarantee Agreement shall be amended as
follows:

          2.01. References in the Note and Guarantee Agreement (including
     references to the Note and Guarantee Agreement as amended hereby) to "this
     Agreement" (and indirect references such as "hereunder", "hereby", "herein"
     and "hereof") shall be deemed to be references to the Note and Guarantee
     Agreement as amended hereby.

          2.02. EXTENSION OF CLOSING DATE DEADLINE. Section 2.04(f) of the Note
     and Guarantee Agreement shall be amended by deleting the date "September
     17, 2001" therein and inserting "October 1, 2001" in lieu thereof.

<PAGE>

          2.03. CERTAIN ADDITIONAL CONDITIONS. Section 3.01(r) of the Note and
     Guarantee Agreement is hereby relettered as Section 3.01(s) and a new
     Section 3.01(r) is hereby inserted into the Note and Guarantee Agreement to
     read as follows:

               "(r) CERTAIN ADDITIONAL CONDITIONS. Evidence satisfactory to the
          Purchasers that:

                    (i) without limiting the generality of the representation
               and warranty set forth in Section 4.01(g) which, pursuant to
               clause (i) of the last paragraph of this Section 3.01 must be
               true and correct as a condition to the purchase of Notes
               hereunder, the destruction of the World Trade Center on September
               11, 2001, and the collapse of neighboring buildings and market
               disruptions as a consequence thereof (and any damage that may
               have been caused to fiberoptic networks of the Borrower and its
               Restricted Subsidiaries in that area) will not result in a
               material adverse effect on the business, assets, results of
               operations, financial condition or liabilities of the Borrower
               and its Restricted Subsidiaries and

                    (ii) subsequent to the date hereof (i.e. September 6, 2001)
               no adverse change in financial, banking or capital market
               conditions has occurred that, in the judgment of each Purchaser,
               could materially impair the ultimate syndication or distribution
               of the Commitments or Notes issued under this Agreement or
               otherwise render the purchase of the Notes inadvisable."

     Section 3. MISCELLANEOUS. Except as herein provided, the Note and Guarantee
Agreement shall remain unchanged and in full force and effect (and, without
limiting the generality of the foregoing, nothing herein shall be deemed a
waiver of the condition set forth in Section 3.01(k) of the Note and Guarantee
Agreement, the parties hereto hereby acknowledging that the due diligence
investigation by the Purchasers of the Issuer and its Subsidiaries referred to
therein has not been completed). This Amendment No. 1 may be executed in any
number of counterparts, all of which taken together shall constitute one and the
same amendatory instrument and any of the parties hereto may execute this
Amendment No. 1 by signing any such counterpart. This Amendment No. 1 shall be
governed by, and construed in accordance with, the law of the State of New York.


<PAGE>


     IN WITNESS WHEREOF, the parties hereto have caused this Amendment No. 1 to
be duly executed and delivered as of the day and year first above written.


                                             METROMEDIA FIBER NETWORK, INC.


                                             By: /s/ Nick Tanzi
                                                 -------------------------------
                                                 Name: Nick Tanzi
                                                 Title: President & CEO


                              GUARANTORS


METROMEDIA FIBER NETWORK                     METROMEDIA FIBER NATIONAL
   SERVICES, INC.                               NETWORK, INC.


By: /s/ Nick Tanzi                           By: /s/ Nick Tanzi
    -------------------------------              -------------------------------
    Name: Nick Tanzi                             Name: Nick Tanzi
    Title: President & CEO                       Title: President & CEO


ABOVENET COMMUNICATIONS INC.                 PAIX.NET, INC.


By: /s/ Nick Tanzi                           By: /s/ Nick Tanzi
    -------------------------------              -------------------------------
    Name: Nick Tanzi                             Name: Nick Tanzi
    Title: President & CEO                       Title: President & CEO


METROMEDIA FIBER NETWORK                     SITESMITH INC.
  OF NEW JERSEY, INC.


By: /s/ Nick Tanzi                           By: /s/ Nick Tanzi
    -------------------------------              -------------------------------
    Name: Nick Tanzi                             Name: Nick Tanzi
    Title: President & CEO                       Title: President & CEO



<PAGE>


MFN OF UTAH L.L.C.                           MFN OF VA, L.L.C.


By: /s/ Nick Tanzi                           By: /s/ Nick Tanzi
    -------------------------------              -------------------------------
    Name: Nick Tanzi                             Name: Nick Tanzi
    Title: President & CEO                       Title: President & CEO



METROMEDIA FIBER NETWORK                     METROMEDIA FIBER NETWORK
   OF ILLINIOS, INC.                            INTERNATIONAL, INC.


By: /s/ Nick Tanzi                           By: /s/ Nick Tanzi
    -------------------------------              -------------------------------
    Name: Nick Tanzi                             Name: Nick Tanzi
    Title: President & CEO                       Title: President & CEO


MFN EUROPE FINANCE, INC                      MFN INTERNATIONAL, L.L.C.


By: /s/ Nick Tanzi                           By: /s/ Nick Tanzi
    -------------------------------              -------------------------------
    Name: Nick Tanzi                             Name: Nick Tanzi
    Title: President & CEO                       Title: President & CEO


MFN JAPAN BACKHAUL, INC.                      MFN PURCHASING, INC.


By: /s/ Nick Tanzi                           By: /s/ Nick Tanzi
    -------------------------------              -------------------------------
    Name: Nick Tanzi                             Name: Nick Tanzi
    Title: President & CEO                       Title: President & CEO


                                             THE ADMINISTRATIVE AGENT


                                             CITICORP, USA, INC., as
                                             Administrative Agent


                                             By: /s/ Richard Zagheb
                                                 -------------------------------
                                                 Name: Richard Zagheb
                                                 Title: Vice President

<PAGE>


                                             PURCHASERS

                                             CITIBANK USA, INC.


                                             By: /s/ Richard Zagheb
                                                -------------------------------
                                                Name: Richard Zagheb
                                                Title: Vice President

                                             MERRILL LYNCH GLOBAL
                                               ALLOCATION FUND, INC.


                                             By: /s/ Lisa Ann O'Donnell
                                                 -------------------------------
                                                 Lisa Ann O'Donnell
                                                 Director, Merrill Lynch
                                                   Investment Managers
                                                 Authorized Signatory

                                             MERRILL LYNCH EQUITY/CONVERTIBLE
                                               SERIES GLOBAL ALLOCATION
                                               PORTFOLIO


                                             By: /s/ Lisa Ann O'Donnell
                                                 -------------------------------
                                                 Lisa Ann O'Donnell
                                                 Director, Merrill Lynch
                                                   Investment Managers
                                                 Authorized Signatory

                                             MERRILL LYNCH VARIABLE SERIES
                                               FUNDS, INC. (MERRILL LYNCH GLOBAL
                                               ALLOCATION FOCUS FUND)


                                             By: /s/ Bryan N. Ison
                                                 -------------------------------
                                                 Bryan N. Ison
                                                 First Vice President, Merrill
                                                   Lynch Investment Managers
                                                 Authorized Signatory

<PAGE>

                                             MERRILL LYNCH SERIES FUND, INC.
                                               (GLOBAL ALLOCATION STRATEGY
                                               PORTFOLIO)


                                             By: /s/ Lisa Ann O'Donnell
                                                 -------------------------------
                                                 Lisa Ann O'Donnell
                                                 Director, Merrill Lynch
                                                    Investment Managers
                                                 Authorized Signatory


                                             JOHN W. KLUGE, CHASE MANHATTAN
                                               BANK AND STUART SUBOTNICK,
                                               TRUSTEES UNDER A TRUST
                                               AGREEMENT BETWEEN JOHN W KLUGE,
                                               AS GRANTOR AND JOHN W. KLUGE AND
                                               MANUFACTURERS HANOVER TRUST
                                               COMPANY, AS TRUSTEES, DATED MAY
                                               30, 1984, AS AMENDED AND RESTATED


                                             By: /s/ Stuart Subotnick
                                                -------------------------------
                                                Name:  Stuart Subotnick
                                                Title: Trustee