1

                                                                     EXHIBIT 8.1

                 [LETTERHEAD OF SMITH, GAMBRELL & RUSSELL, LLP]



                                September 7, 2001


F.N.B. Corporation
One FNB Boulevard
Hermitage, Pennsylvania 16148

Promistar Financial Corporation
551 Main Street
Johnstown, Pennsylvania 15901-1146

         Re:      Agreement and Plan of Merger pursuant to which Promistar
                  Financial Corporation will merge with and into F.N.B.
                  Corporation

Ladies/Gentlemen:

         We have acted as special counsel to F.N.B. Corporation, a Florida
corporation ("FNB"), in connection with the proposed merger (the "Merger") of
Promistar Financial Corporation, a Pennsylvania corporation ("Promistar"), with
and into FNB, pursuant to the terms of and as described in that certain
Agreement and Plan of Merger dated as of June 13, 2001 (the "Merger Agreement")
by and between FNB and Promistar. At your request, and as contemplated by the
Merger Agreement, we are rendering our opinion concerning certain federal income
tax consequences of the Merger. Unless otherwise indicated, all capitalized
terms used in this opinion have the same meaning as used in the Merger
Agreement.

         For purposes of rendering our opinion herein, we have conducted an
examination of the Internal Revenue Code of 1986, as amended (the "Code"), and
such other applicable laws, regulations, rulings, decisions, documents and
records as we have deemed necessary. With respect to factual matters, we have
relied upon the Merger Agreement, including, without limitation, the
representations of the parties set forth therein, and upon certain statements
and representations made to us in certificates by officers of FNB and Promistar,
in each case without independent verification thereof. With the consent of FNB
and Promistar, we have relied on the accuracy and completeness of the statements
and representations contained in such certificates and have assumed that such
certificates will be complete and accurate as of the Effective Time. We have
assumed that any representation or statement qualified by "the best of
knowledge" of the party making such representation or statement, or by any
similar qualification, is correct

   2

F.N.B. Corporation
Promistar Financial Corporation
September 7, 2001
Page 2


without such qualification. As to all matters in which a person or entity making
a representation referred to above has represented that such person or entity
either is not a party to, or does not have, or is not aware of, any plan or
intention, understanding or agreement, we have assumed that there is in fact no
such plan, intention, understanding or agreement with respect to such matters.

         For purposes of this opinion, we have assumed that (i) at least fifty
percent of the outstanding shares of the capital stock of Promistar (the
"Promistar Stock") will be exchanged for FNB Common Stock in the Merger, (ii)
the shares of Promistar Stock constitute capital assets in the hands of each
holder thereof, (iii) the Merger will be consummated according to the Merger
Agreement, and (iv) the Merger will qualify as a statutory merger under
applicable state law.

         Based on the foregoing, and subject to the qualifications set forth
below, we are of the opinion that under the Code:

         (1)      The Merger will constitute a reorganization under Code Section
368(a), and FNB and Promistar will each be a party to the reorganization within
the meaning of Code Section 368(b).

         (2)      Holders of shares of Promistar Stock who exchange such shares
solely for shares of FNB Common Stock will not recognize gain or loss on the
exchange.

         (3)      The federal income tax basis of shares of FNB Common Stock
received in exchange for shares of Promistar Stock (including any fractional
share interest to which the holder may be entitled) will be equal to the
holder's basis of the shares of Promistar Stock surrendered in exchange
therefor, and the holding period of such FNB Common Stock will include the
holding period of the Promistar Stock surrendered in exchange therefor.

         (4)      The receipt of cash in lieu of fractional shares will be
treated as if the fractional shares were distributed as part of the exchange and
then redeemed by FNB, and capital gain or loss will be recognized in an amount
equal to the difference between the cash received and the basis of the
fractional share of FNB Common Stock surrendered.

         The opinions expressed herein are based upon our interpretation of
existing legal authorities, and no assurance can be given that such
interpretations would be followed if the exchange of shares contemplated by the
Merger became the subject of administrative or judicial proceedings. Statements
of opinion herein are opinions only and should not be interpreted as guarantees
of the current status of the law, nor should they be accepted as a guarantee
that a court of law or administrative agency will concur in such statement.

   3

F.N.B. Corporation
Promistar Financial Corporation
September 7, 2001
Page 3


         Except as set forth in paragraphs (1), (2), (3) and (4) above, we
express no opinion with respect to the tax consequences of the Merger,
including, without limitation:

         (i)      the appropriate method to determine the fair market value of
any stock or other consideration received in any sale or exchange;

         (ii)     the state, local or foreign tax consequences of any aspect of
the Merger; or

         (iii)    the federal income tax consequences of any aspect of the
Merger to holders of Promistar Stock who are subject to special tax treatment
for federal income tax purposes, including among others, insurance companies,
tax exempt entities and foreign taxpayers, or to holders of warrants or options
to purchase Promistar Stock, if any, which are exchanged for or converted into
options or warrants to acquire FNB Common Stock.

         The shareholders of Promistar are entitled to rely on the opinions set
forth herein for purposes of approving the Merger. Except as set forth in the
preceding sentence, the opinions set forth herein are addressed only to, and may
be relied upon only by, the addressees hereof, and only in connection with the
transactions contemplated by the Merger Agreement, and are not to be used or
relied upon by any other parties, or in connection with any other transactions,
except with the prior written consent of this law firm.

         We expressly consent to the filing of this opinion as an exhibit to the
registration statement on Form S-4 filed by FNB with the Securities and Exchange
Commission in connection with the Merger, and to the reference to this opinion
in the joint proxy statement/prospectus contained in such registration
statement. In giving this opinion, we do not hereby admit that we are in the
category of persons whose consent is required under Section 7 of the Securities
Act of 1933, as amended.

                                          Very truly yours,

                                          SMITH, GAMBRELL & RUSSELL, LLP

                                          /s/ David W. Santi

                                          David W. Santi