EVER SINCE the National Gallery of Art opened its East Building in 1978, it has been quietly improving its 20th-century collections. Eight recent acquisitions--pictures by Frank Stella, Roy Lichtenstein, Helen Frankenthaler and Picasso, and sculptures by Brancusi, Tony Smith and Joseph Cornell--have now been placed on view there.

All have come to the museum in the past three years. One, a 1926 Picasso, was bought by the museum. Its price was not disclosed. The others came as gifts.

The newest of these objects, a gift from Lila Acheson Wallace, is "Jarama II," a brilliantly colored, exuberantly complex "race track" picture by Frank Stella completed just last year. Part painting for the wall and part bolted sculpture, it is 10 feet high and eight feet wide and more than two feet deep. Its dazzling design is based upon the straightaways and hairpin turns of an automotive race track. Its materials, too, suggest the lightness and the speed of a space-age racing car--it's made of cut-out forms of painted, etched magnesium.

A second major Stella, a picture strong enough to take on the building's skylit garden court, has been installed nearby. This one, on stretched canvas, is "Sacramento Moposol" (1974) from the artist's series of concentric squares. It was given to the Gallery by the Collectors Committee.

Roy Lichtenstein's "Cubist Still Life" (1974) is another gift from Wallace. The picture, with its fractured guitar and hand-painted wood graining, wittily recalls the early 20th-century still lifes of Juan Gris--and the Gallery's new Picasso, a large collage on wood dated 1926. The Picasso, bought from the artist's estate with monies from the Gallery's Chester Dale Fund, is called "Guitar." Its guitar looks oddly African. Though most of it is painted, its strings are made of real string tied around real nails. It is one of those rare Picassos, influenced by Surrealism, that reveals, in Andre' Breton's phrase, "a rapprochment between collage and the idea of violence."

Another recently acquired picture, an anonymous gift, is "Wales" (1966) by Helen Frankenthaler.

"Untitled (Medici Prince)" is the first Joseph Cornell box to come to the museum. This piece of "poetic theater" stars a young boy with a sword, a kind of premature adult, borrowed by Cornell from a 16th-century Italian Mannerist portrait by Sofonisba Anguissola. The box, another gift from the Collector's Committee, is a 1950s variant of Cornell's "Object (Medici Slot Machine)" of 1942.

"Wandering Rocks" (1967), Tony Smith's five-element bronze sculpture, is another gift from the Collector's Committee. An extraordinary piece, installed on the Gallery's south lawn, it is a sort of minimalist mystery, whose tetrahedral forms seem to move as one moves around them.

Constantin Brancusi's "Maiastra" (c. 1911), a gift from Katharine Graham, is a bronze that precedes, and predicts, the master's famous "Bird in Flight."

One new acquisition is not yet on view. The Gallery, with funds from the Andrew W. Mellon Fund and the Ailsa Mellon Bruce Fund, has purchased "Shirtfront and Fork," a funny-formal 1922 wood relief by Jean Arp, which is to be included in "Arp: The Dada Reliefs," a National Gallery exhibition that will open July 3. The price was not disclosed.