After a year of "quality" films provided free by the public library but scorned by their viewers, the prisoners at the Prince George's County detention center - with the help of federal law enforcement funds - are now getting their preferred choice of cinematic sex and violence.

They have gone from movies like "The Autobiography of Miss Jane Pittman" "Brian's Song" and "Bad Day at Black Road" to R-rated "Willie Dynamite" who, according to the film rental catalogue, is "a cold man who'll kill and destroy to stay on top."

At the District's correctional center in Lorton, Va., however, there's a contrary trend toward "more movies of substance," according to one prison official.

Meanwhile, at the Fairfax County jail, the 190 prisoners get a twice weekly dosage of religious and ethical movies, courtesy of a Georgia gospel film, company and the Billy Graham people.

They are all captive audiences, these incarcerated movie watchers, wrenched by the criminal justice system from the violence of the streets into the sometimes tense and always tedious prison atmosphere. It's a life where almost any break in the rarely changing routine is welcome.

In Prince George's said film program coordinator Hedda Sachs, "The purpose is more to help relieve tensions, help them escape from reality for a couple of hours."

The film program at the Upper Marlboro jail began in 1975 with one a month from the public library. Included were such screen classics as "The Oxbow Incident" and "The Hustler," among others. They bombed.

"Although the films were of high caliber, it was not really what our population wanted to see," said Sachs. "They like violence, they like sex, they like fast-moving movies." Many are accompanied by cartoons.

So last November, with $1,5000 in federal funds, Sachs began ordering what the prisoners wanted from Swank, a national film rental firm. The movies - many of them black-oriented - have been playing every two or three-weeks to attentive inmates.

"I thought 'Jesus Christ Superstar' would really be great around Christmas time," Sachs said, but Swank people told her to forget it. Instead, the inmates are watching "Blacula," "The Valachi Papers," and "Slaughter," in which Jim Brown plays an ex-Green Beret who crosses "paths with Mafia henchmen, government agents and beautiful ladies."

"The worst we get is "R" rated films, Sachs said. "It would be cruel and unusual punishment to show men locked for months on end X-rated films. Rs are at least suggestive. They can handle that, but some of the X-rated films could cause problems in the cell-blocks."

Sachs said she is not worried that the violence in the films might create problems in the cellblock. "They think (the violence) is funny and they can watch all the violence they want on television, anyway," she said.

At Lorton, they've gone through a "transitional period," according to Salanda Whitfield, acting superintendant. A while ago, "wild action movies" were in vogue. But a year ago, Whitafield said, prisoners complained "they wanted movies of substance, movies other peoplw are enjoying in the community. We were kind of surprised.

Lorton's 1,300 inmates are treated to movies three nights a week. Recent movies have included such generally well-received films as "Bad News Bears," "Front Page" and "The Hindenberg," as well as the likes of "Combat Cops," "Cuthroats" and "Capt. Apache."

At Fairfax jailM there's no money to rent films but $650 from commissary profits paid for projector and screen that are moved from cellblock to cellblock. The emphasis is on religion, social issues and morality, according to Chaplain Lacey Justice. Only about half the inmates watch the films, he said.

There are no R-rated films, much less X-rated. Violent films, said Mr. Lacey, "tend to reinforce the type of life they have been living outside by exposing them to it. My feeling is they can watch violence on TV. From my vantage point, to show that type of film would be a waste of time."