THE UNCUT VERSION of Bernardo Bertolucci's 1900 will be shown at the National Gallery's East Building auditorium, Saturday at 2. In this lavish 1976 film, Bertolucci -- most famous for Last Tango in Paris, but who also made The Conformist and Luna -- covers a half-century relationship between a landowner's son and a peasant boy, who are divided by class but united in friendship. Bertolucci traces them through two world wars, culminating in the liberation of Italy in 1945. The (free) film lasts 5 1/2 hours and will have one intermission.

For dinosaur freaks (and we know you're out there, fantasizing about Mesozoic creatures with frilly backs), there will be a free screening of Valley of the Gwanji at noon this Friday at the National Museum of Natural History's Baird Auditorium. This animated 1960 flick about the big, the bad and the herbivorous also will be shown Saturday and Sunday at 1.

Free films at the Mary Pickford Theater: Monday at 7:30, it's Gideon's Trumpet, a 1980 CBS movie produced by John Houseman, which recounts the landmark court case, Gideon v. Wainright, in which the Supreme Court -- reversing itself -- established the rights of defendants to legal counsel. Henry Fonda stars as Clarence Earl Gideon, who was convicted of breaking into a poolroom in 1961, but who petitioned the Court successfully with handwritten letters from his jail cell. Wednesday at 7:30, the theater will show Stanley Donen's Indiscreet, starring Cary Grant and Ingrid Bergman. The theater is on the third floor of the Library of Congress' James Madison Building. Call 287-5677 for reservations . . .

Thursday at 6:30 (and Saturday August 29 at 6:30), the American Film Institute will premiere Scottish filmmaker Michael Hoffman's Restless Natives. In this 1986 McRobin Hood film, a couple of looney Scots rob tour buses with pop guns full of sneezing powder and distribute the loot to the poor . . .

Also Thursday at 7, free, the National Archives' fifth floor theater will screen The Caine Mutiny, with Humphrey Bogart, Jose Ferrer and E.G. Marshall.