FIFTY YEARS AGO this weekend, Neighborhood Theaters Incorporated opened the 830-seat State theater in Falls Church. NTI still operates the house, and this weekend celebrates the theater's golden anniversary with a 50-hour movie marathon.

In November the theater reduced its general admission price to 99 cents per flick, but beginning on Friday evening at 8, 99 cents buys a ticket for 26 non-stop movies for those who can sit that long. Blankets will be permitted but no sleeping bags will be allowed in the theater. The marathon begins with the Eddie Murphy trilogy, Beverly Hills Cop, Trading Places and 48 Hours, followed by the Friday the 13th series, the Star Trek series, both of The Godfather movies and more through 10 p.m. Sunday. The theater is at 220 North Washington Street in Falls Church. For a showtimes schedule, call 532-1555.

The State isn't the only area theater coming down with 99-cent fever. In Prince George's County, Paul Sanchez' theaters have flourished where others have failed. The 36-year-old theater operator figured volume was the best road to success at his two largest houses, the 970-seat New Carrollton and the 815-seat Riverdale, where admission is 99 cents for all showings.

"I went to $2 at the Riverdale because I couldn't compete with the AMCs at Beltway Plaza and in the Carrollton Mall. That improved business quite a bit," says Sanchez. "Then I went to 99 cents to see what that would do. Then business really took off!"

Sanchez, who also operates the Allen 1 & 2 and the Flower 4 Cinemas, started sub-leasing the New Carrollton theater from Neighborhood Theaters Incorporated in mid-November and has been selling tickets for 99 cents since.

Down the Beltway a couple of miles at Central Avenue, The Hampton Mall Twins begin their third week with the 99-cent ticket and have tripled their business since cutting the $3 admission charge.

Meanwhile, Roth Theaters will take on the Chicago Bears and the New England Patriots this Sunday. While the two professional football teams test their gridiron skills, all but one of the Roths theaters, the Roth's Village, will sell tickets for $1. See the Movie Directory for details.

Speaking of the gridiron, not all the stars at the 51st Washington Touchdown Club's annual awards banquet last Saturday evening were from the playing field. Goldie Hawn, who was born and reared in Takoma Park, was on hand to present to an award from the club to Bears running back Walter Payton.

Hawn was also there to pick up an honor of her own, the "Local Personality Makes Good Award." Actors seldom miss a chance to plug their latest film, and Hawn was no different. She showed the crowd clips from her latest movie, Wildcats, in which she plays a coach of a football team full of inner-city rowdies. The film opens February 14.

Hawn was followed by Gene Autry, "the Singing Cowboy," who appeared in more than 60 singing westerns produced by Republic. The 78-year-old Autry said he "stopped making films some years ago when they started making the horses taller."

Autry, who owns the California Angels baseball team, was awared the Touchdown Club's Hubert H. Humphrey Humanitarian Award "for outstanding contributions to sports, America, and the world."

The Library of Congress, keeper of more than 3,000 restored feature films made before 1912, will present film historian Kemp Niver in a question-and-answer session following a screening of turn-of-the-century movies on Tuesday afternoon at 4 in the Mary Pickford Theater.

Niver won an Academy Award in 1954 for developing a technique of transferring aging movies from contact paper to film, frame by frame. Before 1912, moviemakers were not able to obtain copyrights for films, so they made contact paper prints directly from the original motion picture film. The paper prints could then be issued copyrights. Niver is credited with saving those days of early filmmaking with his restoration process.

The Library of Congress, with the help of the Academy of Motion Picture Arts and Sciences, worked from 1953 to 1964 copying those films. In 1967, Niver published a catalogue of the films called "Early Motion Pictures: The Paper Print Collection in the Library of Congress." Recently reissued, the book sells for $24 plus tax at the Library.

Patrick Loughney, the Library of Congress' curator of motion pictures, will speak on the early days of moviemaking in a free lecture January 31 at Catholic University. He will look at the unbroken connection between late 19th- century stage entertainment and the earliest days of filmmaking, and examine Thomas Edison's and the Biograph Company's films produced from 1899 to 1905. He'll also screen a reel of vintage short features. The lecture begins at 4 p.m. in CU's University Center West Wing, Rooms 309-311. Call 635-5794.

If you've watched MTV at all, you've probably wondered how hard could it possibly be to shoot a music video. Find out this Sunday at 7 with a screening and discussion period as Robert Starbird presents the Washington premiere of his Not Lonely, a 41/2-minute production starring The Casuals, a Charlottesville- based rock'n'roll group featuring Johnny Sportcoat. Also highlighted will be Daniel Rainey's two longer format music videos, Tippin' at Matt Kane's and The Seldom Scene, from a live concert at the Birchmere. The program is free at the George Mason University Law School, Metro Campus conference room, 3401 North Fairfax Drive, Arlington. Call 234-3754 or 243-1276.

SHORT SUBJECTS -- The "American Lives" series at the National Archives offers the 1985 portrait of a religious community, The Amish: Not to be Modern, on Friday at noon. The free series continues next Thursday evening at 7 with the 1940 film of Thorton Wilder's Pulitzer Prize-winning play Our Town.

The Alexandria campus of Northern Virginia Community College continues its free "Film Watchers" series Friday evening at 8 with the 1979 Japanese production Vengeance Is Mine in the Bisdorf Building, Room 110. Call 845-6207.

The American Film Institute's "Celluloid Sleuths" series this weekend screens Michael Shayne, Private Detective double billed with The Man Who Wouldn't Die on Friday at 8:30 and Sunday at 5:30. On Saturday at 7 and Sunday at 8:30 it's the 1969 production of Marlowe starring James Garner, Gayle Hunnicut and Rita Moreno. Call 785-4600.

The Smithsonian Resident Associate Program's Festival of Indian Films screens the 171-minute Tarang on Monday at 7:30 in the American History Museum's Carmichael Auditorium. Call 357-3030 for reservations . . . On Wednesday at noon the museum's free "America on Film" series shows two 1934 films in Carmichael Auditorium, Them Thar Hills and Six of a Kind.

Jean Cocteau's 1930 surrealist Le Sang d'un Poete screens for the benefit of the Washington Review of the Arts on Wednesday at 7:30 p.m. at the French Chancery, 4101 Reservoir Road NW. Tickets are $10. For reservations, call 638-0515. On Thursday at 5:30 and 8 see The Testament of Orpheus on the Chancery screen. Cost is $3 each session. Call 944-6400.