LOST PICASSOS. Moved from their usual spot in the West Building of the National Gallery, five Picassos now are to be found in the East Building, in a survey of the Gallery's 20th-century art collection.

Gathered together for the first time in contiguous galleries on one level are the Gallery's School of Paris paintings and sculpture, as well as its collection of American modernists and abstract expressionists. They've come over to join some East Building landmarks: Robert Motherwell's massive "Reconciliation Elegy," which was commissioned for the building, as well as the big Matisse cutout that used to be upstairs in the "tower," and a model for the Calder mobile.

It was necessary to move things around to make room for the "Treasure Houses of Britain" exhibition due in November. The new configuration has a serendipitous flow. Some of the Americans have even been given separate rooms -- the raucous cartoons of Roy Lichtenstein do not disturb the serene floating rectangles of Mark Rothko.

But inadvertently the new installation also brings together the Gallery's most misunderstood art, if the overheard comments of visitors are any test:

Giacometti's bronze sculpture, "The Chariot," was pronounced by one young woman to be "Anorexic Boy on a Bike."

In the Robert Rauschenberg room, a woman pointed to the weathered board that forms the base of "Red Rock," a mixed- media construction, and said to no one in particular, "Gee, I've got a lot of art in my backyard."

Even Joan Miro did not escape comment. Referring to the surrealistic, gestural lines of "Head of a Catalan Peasant," a girl said, "This is famous? I can draw that."

"The thing is, Christine," her mother explained patiently, "that this person thought of it when nobody else had thought of it. That's what made it special."

In the Georgia O'Keeffe room, her series "Jack in the Pulpit," on loan from a private collection, is especially compelling, and typically vaginal. Here, a budding young artist was heard to ask her mother, "Georgia? That's a girl's name?"

And then there was the man who walked into the room where five Rothkos are hanging, looked at the horizontal bars painted by the minimalist artist, and said simply, "I'm sorry."

INSTALLATION OF THE 20TH CENTURY COLLECTION -- At the National Gallery of Art's East Building, through April 1986.