The Fairfax County Library Board, bowing to pressure from the county's Board of Supervisors, has agreed to require that borrowers of R-rated video cassettes show identification proving that they are at least 17 years old.

The policy, which went into effect yesterday, came a week after the county board criticized the library system for allowing minors to check out cassettes of such R-rated films as "Body Heat" and "Halloween."

"Certainly they the supervisors hold the purse strings, and we felt that this was the wise decision to make," said June Picciano, chairman of the library board.

"We were looking at our total system, and I think some of that might have been in jeopardy had we not chosen to restrict the tapes ."

Meanwhile, Democratic Supervisor Martha V. Pennino of Centreville, who has led the criticism of the tapes, said yesterday that she hoped the library board would stop buying the R-rated tapes, which she said are a threat to the morals of young people.

Pennino said she believes that the purchase of the videocassettes is an improper use of taxpayers' money.

"If they want it spelled out in detail by the board of supervisors, I'll be happy to put the motion before the board. But I hope it won't be necessary," Pennino said.

If the library continues purchasing the tapes, Pennino said "I think the board would think about taking serious action." She said the board could withhold county library funds, as well as seek the resignations of members of the library system's governing body.

Library officials in April had refused to impose age restrictions on borrowers, reaffirming a previous policy which guaranteed all county citizens free access to all library materials, including the R-rated tapes.

About 10 percent of the library system's collection of 1,200 tapes carry "R" ratings, and none are rated "X", according to library officials. The collection is valued at approximately $70,000.

The debate in Fairfax mirrors similar disputes across the country, as libraries struggle to define their role as information providers in an increasingly technological marketplace.

The American Library Association has recommended against restrictions on any library materials including videotapes, arguing that such limits can mark the first step toward censorship. Others argue, however, that tapes must be restricted because they are far more graphic than books and thus more harmful to impressionable youngsters.

Fairfax Library Director Edwin S. Clay said the new policy did not mark a step toward censorship of library materials, and said its practical effect would be almost negligible. "We have had very few people who looked like minors waiting to check out R-rated films," Clay said.

Three other area jurisdictions have imposed age restrictions on video-tape borrowers. In Arlington and Prince George's counties, borrowers must be at least 18. Borrowers of R-rated tapes in Alexandria must prove that they are at least 17. Montgomery County has no restrictions, and D.C. has no R-rated cassettes available for borrowing.