EXHIBIT 4.1

                             SUPPLEMENTAL INDENTURE

     SUPPLEMENTAL  INDENTURE,  dated as of the 10th day of October, 1997, to the
Indenture (the  "Indenture")  dated as of November 1, 1995, by and between Porta
Systems Corp., a Delaware  corporation (the "Company"),  as issuer, and American
Stock Transfer & Trust Company (the "Trustee"), as trustee.

                              W I T N E S S E T H:

     WHEREAS, the Company and the Trustee entered into the Indenture dated as of
November 1, 1995 in connection  with the issuance of the  Company's  Zero Coupon
Senior Subordinated Convertible Notes due January 2, 1998 (the "Notes"); and

     WHEREAS,  pursuant to the Indenture, the Company has issued and outstanding
Notes in the principal  amount of  $26,156,365.00,  and additional  Notes in the
maximum  principal  amount of  $1,473,062.00  may be issued in exchange  for the
Company's 6% Convertible Subordinated Debentures Due July 1, 2002; and

     WHEREAS, the Company desires to modify the Conversion Price of the Notes as
hereinafter provided; and

     WHEREAS,  the  modification  of  the  Conversion  Price  pursuant  to  this
Supplemental Indenture is permitted by the Indenture;

     WHEREFORE, the Indenture is hereby modified as follows:

     1.  All  terms  defined  in the  Indenture  and  used in this  Supplemental
Indenture shall have the same meanings in this Supplemental  Indenture as in the
Indenture.

     2. Shares of Common Stock issued upon  conversion of the Notes  pursuant to
the terms of this  Supplemental  Indenture  are  referred to as the  "Conversion
Shares."  The person  who  either  (i) owns the Notes or (ii) is the  beneficial
owner of Notes held in the name of a nominee or (iii) is a Permitted Transferee,
as hereinafter defined, is referred to as a "Converting Noteholder."

     3. If any Note is tendered for  conversion  on or subsequent to the date of
this  Supplemental  Indenture,  subject to the provisions of Paragraph 6 of this
Supplemental  Indenture,  the  following  terms shall apply with respect to each
such conversion.

          (a) The Conversion  Price shall be reduced to three and 65/100 dollars
($3.65).

          (b) No fractional  shares of Common Stock shall be issued.  Cash shall
be paid in lieu of  fractional  shares  as  provided  in  Section  15.03  of the
Indenture.

          (c) All  certificates  for Conversion  Shares issued to the Converting
Noteholders (other than Converting Noteholders of record who do not identify the
beneficial  owner of the  Conversion  Shares  within  thirty (30) days after the
Notes are  presented for  conversion)  pursuant to this  Supplemental  Indenture
shall bear the following legend:


                                      -1-
<PAGE>

     "THE SHARES OF COMMON STOCK  REPRESENTED  BY THIS  CERTIFICATE  WERE ISSUED
     UPON CONVERSION OF THE ISSUER'S ZERO COUPON SENIOR SUBORDINATED CONVERTIBLE
     NOTES DUE JANUARY 2, 1998. UNDER CERTAIN  CONDITIONS,  ADDITIONAL SHARES OF
     COMMON STOCK MAY BE ISSUED TO THE BENEFICIAL OWNER OF THESE SHARES (BUT NOT
     TO ANY  TRANSFEREE  OTHER THAN A PERMITTED  TRANSFEREE) AS SET FORTH IN THE
     SUPPLEMENTAL INDENTURE DATED AS OF OCTOBER 10, 1997, BETWEEN THE ISSUER AND
     AMERICAN STOCK TRANSFER & TRUST COMPANY, AS TRUSTEE."

          (d)  In  the  event  of  any  transfer  of a  Converting  Noteholder's
Conversion Shares, other than to a Permitted Transferee, the legend set forth in
Paragraph  3(c)  of this  Supplemental  Indenture  shall  not be  placed  on the
certificates issued to the transferee,  and no Contingent Shares, as hereinafter
defined, shall be issued with respect to such Conversion Shares.

          (e) A Permitted Transferee shall mean a transferee which is either (i)
the estate of the  Converting  Noteholder  or (ii) a  transferee  of  Conversion
Shares pursuant to a transfer which reflects a transfer of record ownership only
and does not reflect any change in the identity of the  beneficial  owner of the
Conversion Shares.

     4. Subject to Paragraph 5 of this Supplemental Indenture, additional shares
(the  "Contingent  Shares") of Common  Stock  shall be issued to the  Converting
Noteholder  with respect to the  Converting  Noteholder's  Retained  Shares,  as
hereinafter  defined,  in the  event  that the  Anniversary  Price,  hereinafter
defined,  is less than $3.65, on the terms and conditions  hereinafter set forth
in this Paragraph 4.

          (a) The  Anniversary  Price shall mean the average  closing  price per
share of Common  Stock on the  principal  stock  exchange or market on which the
Common Stock is traded for the sixty (60) trading days immediately preceding the
date  which is one  year  from  the  Effective  Date,  as  hereinafter  defined;
provided,  however,  that if, on any date,  there is no  trading  in the  Common
Stock, the average of the closing bid and asked prices shall be used.

          (b) (i) The  Effective  Date  shall  mean the date  that the  Required
Percentage of Notes have been tendered for conversion.  The Required  Percentage
shall mean the percentage of the outstanding principal amount of Notes (based on
the  principal  amount  of Notes  outstanding  on the date of this  Supplemental
Indenture) which must be tendered for conversion in order for this  Supplemental
Indenture to become effective.  The Required  Percentage shall be 95%; provided,
however, that the Company may, in its sole discretion, on notice to the Trustee,
reduce the Required Percentage to a percentage which is not less than 85%.

               (ii)  Notwithstanding the provisions of Paragraph 4(b)(i) of this
Supplemental  Indenture,  the Required  Percentage  shall be deemed to have been
attained if at least 70%, but less than 85%, of the outstanding principal amount
of Notes shall have been  tendered  for  conversion  and the Company  shall have
delivered to the Trustee an Officers'  Certificate  stating that the Company has
the ability to pay, and agrees to pay, the  principal  amount of all Notes which
have not been tendered for  conversion,  either  through  available  cash,  then
existing and in-place lines of credit or irrevocable  written  commitments  from
financially  responsible  parties with a  demonstrated  capability  to fund such
irrevocable written commitments.  If subsequent to the delivery of the Officers'
Certificate  referred to in this Paragraph 4(b)(ii),  the Required Percentage is
attained  pursuant to Paragraph  4(b)(i) of this  Supplemental  Indenture,  this
Paragraph 4(b)(ii) shall no longer apply.


                                      -2-
<PAGE>

          (c) Retained Shares shall mean Conversion  Shares that (i) were issued
to the  Converting  Noteholder  upon  conversion  of the Notes  pursuant to this
Supplemental  Indenture,  (ii) were not  transferred  subsequent  to the initial
issuance other than to a Permitted  Transferee,  and (iii) were held by either a
Converting  Noteholder or a Permitted  Transferee  continuously from the date of
conversion  to the  Anniversary  Date.  No  Contingent  Shares will be issued in
respect of any Conversion Shares which were sold or otherwise  transferred prior
to the Anniversary  Date other than to a Permitted  Transferee.  The Company may
request documentary evidence with respect to any transfer which purports to be a
transfer to a Permitted Transferee.

          (d) In the event  that the any  Converting  Noteholder  engages in any
short sales of Common Stock  (including short sales against the box), the number
of  Retained  Shares  shall be reduced  by the number of shares of Common  Stock
subject to such short sales or sales against the box.

          (e)(i) The number of  Contingent  Shares  issuable with respect to the
Retained  Shares  of  any  Converting  Noteholder  shall  be  determined  by the
following formula:

     Contingent Shares = (Retained Shares / N) x B x (1 - (P - $2.65))

     In the foregoing formula:

N    shall be the number of shares  determined by dividing the principal  amount
     of Notes  outstanding  on the Effective Date prior to any conversion of any
     Notes pursuant to this Supplemental Indenture by $3.65.

B    shall be the number of shares of Common Stock  determined by  subtracting N
     from 8,000,000.

P    shall be the greater of the Anniversary  Price or $2.65. If the Anniversary
     Price is equal to or greater  than $3.65,  no  Contingent  Shares  shall be
     issued.

               (ii) In case the Company shall after the date of  conversion  (A)
pay a dividend or make a distribution on its shares of Common Stock in shares of
Common Stock,  (B) subdivide,  split or reclassify its outstanding  Common Stock
into a greater number of shares, (C) effect a reverse split or otherwise combine
or reclassify its outstanding  Common Stock into a smaller number of shares,  or
(D) issue any  shares by  reclassification  of its shares of Common  Stock,  the
amount of Contingent  Shares which may be issued and the related  dollar amounts
contained in this paragraph (e) will be adjusted accordingly.

          (f) Prior to the issuance of any  Contingent  Shares to any Converting
Noteholder,  the Trustee and the Company shall have received from the beneficial
owner a statement  to the effect  that,  except as  expressly  set forth on such
statement,  (i) the  Conversion  Shares  beneficially  owned  by the  Converting
Noteholder have been beneficially owned by the Converting  Noteholder (including
any Permitted Transferees) for an uninterrupted period of time commencing on the
date of the  conversion  of the  Notes  until  the  Anniversary  Date,  (ii) the
Converting  Noteholder  has not  engaged  in any  short  sales of  Common  Stock
(including short sales against the box), and (iii) the Converting Noteholder has
not assigned or  transferred  any  beneficial  interest in any of the Conversion
Shares.


                                      -3-
<PAGE>

     5. No Contingent  Shares shall be issued in the event that,  for any period
of twenty (20)  consecutive  trading  days during the period  commencing  on the
Effective Date and ending on the day prior to one year from the Effective  Date,
the average closing price of the Common Stock on the principal stock exchange or
market on which the  Common  Stock is traded is at least  $3.65.  In making  the
foregoing computation, if, on any date, there is no trading in the Common Stock,
the average of the closing bid and asked prices shall be used for such date.

     6. In the event that the Required Percentage of Notes shall not be tendered
for conversion by November 7, 1997, this Supplemental  Indenture shall terminate
and have no force or effect,  and the Indenture shall continue in full force and
effect  as  if  it  had  not  been  amended  by  this  Supplemental   Indenture.
Notwithstanding  the  foregoing,  the Company shall have the right,  in its sole
discretion to postpone the date by which the Required  Percentaged of Notes must
be converted on one or more  occasions from November 7, 1997 to a date not later
than  December  1, 1997,  which  date may be  extended  with the  consent of the
holders of a majority of the  principal  amount of Notes which had been tendered
for conversion.

     7. Each Note  shall,  without  any  action  on the part of the  holder,  be
entitled the benefits of this Supplemental Indenture.

     8. Except as amended by this  Supplemental  Indenture,  the Indenture shall
continue in full force and effect.


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

                                       PORTA SYSTEMS CORP.

                                       By:_____________________________________
                                          William V. Carney, Chairman and
                                            Chief Executive Officer

                                       AMERICAN STOCK TRANSFER &
                                       TRUST COMPANY

                                       By:_____________________________________
                                                           , Authorized Officer

                                      -4-