The Motion Picture Association of America announced yesterday that it is reforming its system of rating films, effective immediately, abandoning its controversial X rating in favor of a new category, to be called NC-17.

The new category, like the X rating, will forbid admission to anyone under the age of 17. The letters NC stand for "No Children."

MPAA President Jack Valenti said yesterday that the association will trademark the new rating, meaning that it can be applied only by the MPAA "to those films submitted to the rating system." The old X rating was not trademarked and was widely employed in promotions by the adult-movie industry, which gave the X a stigma that NC-17 is designed to eliminate.

In a statement, the MPAA said, "We have concluded that over the years some people have come to endow the X film rating with meaning it does not have, never has had and was not intended by founders of the rating program."

The statement, which was cosigned by representatives of the National Association of Theater Owners, also announced a second revision in the system, providing for "brief explanations as to why a film received an R rating to film critics, theater owners and video dealers." This second reform will not involve any changes in the criteria for the R rating or in the labeling of R-rated films, and the explanations will not be included in advertising. An R rating mandates that moviegoers under 17 will not be admitted unless accompanied by an adult. Roughly one-third of all movies produced by the major studios receive R ratings.

These explanations of R ratings will begin to be circulated within the next two weeks, the MPAA said.

The changes in the system come after months of agitation from within the film business over growing concerns that the X rating unfairly stigmatized certain films with adult content as pornographic. A number of recent movies from independent distributors, including Peter Greenaway's "The Cook, the Thief, His Wife & Her Lover" and Pedro Almodovar's "Tie Me Up! Tie Me Down!," received X ratings, and rather than attempt to market the pictures on those terms, their distributors released them unrated.

Russell Schwartz of Miramax Films, which released both "The Cook, the Thief" and "Tie Me Up!" and brought suit against the ratings board over the latter's X rating, yesterday applauded the MPAA's action. "This leaves the X rating with the pornographers, where it belongs," he said. "It means that the system will now function as it was originally intended to."

By removing the X rating, the restrictions that newspapers place on advertising X-rated films and that theater owners place on showing them are also in effect removed. But Schwartz added that distributors' problems may not be over: Some communities may well restrict advertising and the availability of theater screens for NC-17 movies. "But they certainly can't call these films pornographic anymore," he said.

Universal Chairman Tom Pollock, whose upcoming "Henry and June" received an X from the ratings board, said yesterday from Los Angeles that he was "very pleased" with the MPAA decision. "I think it will be a benefit both to the industry and the consumer," he commented. He also announced that "Henry and June," which is coming out Oct. 5, will be the first film to be released with an NC-17 rating.

Asked what the revisions signified in terms of real change, Pollock said, "Not much." The economics of the business, he said, don't allow for too many films to carry the NC-17 rating. However, he said, those that do carry it will have "a rating with honor, rather than a rating with dishonor."

Philip Kaufman, director of "The Unbearable Lightness of Being" and "Henry and June," also applauded the action. "A fresh, bold step was needed, and Jack Valenti and the MPAA made it," he said from his San Francisco office.

Los Angeles Times film critic Peter Rainer, chairman of the National Society of Film Critics, which had urged the MPAA to reform the ratings system, voiced approval of yesterday's action. "As far as the MPAA goes, I think this is good news," he said. "By creating a new category that does not have the stigma of the X, it will potentially create a situation where filmmakers are now able to explore adult themes without de facto censorship."

Most of this year's spate of X ratings have gone to independent releases. There had been speculation that the X for "Henry and June" would increase industry pressure for ratings reforms. Valenti dismissed that speculation as "total nonsense."

"You can't make a move like this based on just one picture," he said. "And I'm sure Tom Pollock understands that, because I never received one call from him. Not one."

Schwartz, however, contended that by making these new changes, the MPAA avoided a potentially embarrassing situation. "The ratings appeal for 'Henry and June' was scheduled for October 3," he said, "and the release for October 5. I think the timing of these changes speaks for itself."

The rating system, which the industry established in 1968 to stem increasing pressure from religious and special-interest groups, has been amended only once before. That was in 1984, after the release of several films, including Steven Spielberg's "Indiana Jones and the Temple of Doom," that seemed to stretch the definition of the PG rating. At that time the association added the PG-13 category, advising parents of material that might be too intense for younger viewers.

Some critics of the ratings system had urged that it be updated through the addition of a new rating -- an A (Adult) category had been suggested -- between R and X to differentiate between films of serious artistic aspirations and those that are considered exploitative. Valenti, speaking from the MPAA's offices in New York, said he has been "quite unambiguous" in his refusal to do so. But after months of "struggling" with the issue, he said, he determined that the criticism directed at the X rating, especially over the "patina" it had taken on, had "some validity." He said that he knows the new rating, too, may become "soiled" and that he doesn't know whether these reforms will fully satisfy the system's critics.

"I expect criticism to continue," he said. "I do know, however, that what I'm doing is in the long-range best interests of an enduring and useful ratings system."