"This museum is just like the Freer used to be: Nobody's in it!? said one contented lunch-hour visitor to another -- the only other, as it turned out.

They were in the Museum of Modern Art of Latin America, opened in 1976 as a Bicentennial gift to the people of the United States from the Organization of American States. To date, the gift has remained virtually unnoticed.

"It's true," says Rafael Sarda, public relations coordinator for the museum, "most people don't know we're here." Closed in recent months for refurbishing and reinstallation of its greatly expanded collections, the museum has just reopened with new carpeting, soft benches and twice as much art. "This time, people are bound to notice," says Sarda.

Installed in the handsome two-story residence built in 1910 by Andrew Carnegie to house secretaries-general of the OAS, this cool, homey oasis is situated just off the Mall at 201 18th St. NW behind the Pan American Union Building. The museum now includes 450 works surveying the leading 20th century, artists of Latin America, ranging in style from semi-abstraction, expressionism and surrealism to geometric, kinetic and Op. The art is of the highest order, thanks to knowledgeable acquisitions committees that have included, among others, Joshua Taylor director of the National Collection of Fine Arts and Collector David Lloyd Kreeger.

"This is," museum director Jose Gomez-Sicre says, "a museum that has few equals in the Western Hemisphere. And it was necessary because, though contemporary artists of Latin America are represented in many U.S. museums, their works are too seldom featured and rarely shown in a group -- as evidence of a solidly established, continent-wide movement."

To this end, the OAS began 20 years ago to exhibit the work of contemporary Latin American artists in a small gallery at the rear of the palm-filled atrium in the main building at 17th Street and Constitution Avenue. It was widely known for the beautiful parrots sheltered, until the current OAS Secretary General Orfila banished them to the National Zoo, proclaiming that he was tired of people thinking of Latin America as nothing more than exotic birds, big hats and bandilleros.(Even Jackie Onassis complained at the removal of this Washington institution.) Botero had his first American show here, as did Cuevas and hundreds more.

In 1957, OAS began systematically purchasing works of art, which remained largely in storage until the museum opened in 1976 with 180 works. Since then, through gifts and purchases, the museum collections have burgeoned to the present number. The new installation gives the art more shape and more force than it revealed before.

The first gallery is devoted to fore-runners -- wellknown early masters such as Mexican painters Jose Clemente Orozco and Rufino Tamayo, each represented by rare works newly placed on anonymous loan. Orozco's powerful "Dance in the Bordello," from his pre-Revolution, pre-mural days, is in an expressionist style that seems to have evolved early and, possibly, independently of the German Expressionists. h"There is no work by Orozco from this period on view in any other public collection," boasts Gomez-Sicre.

There are powerful paintings by Rodrigo Penalba and Armando Morales of Nicaragua, Matta of Chile (now in exile in France) Alejandro Obregon and Enrique Grau of Colombia, the later represented by a surrealistic nude man standing under an umbrella. The puffed face here recalls Botero, though an absence of dating and research on the paintings, here as elsewhere, leaves viewers not knowing which came first. (Botero, who received a major show at the Hirshorn last winter, is one of the few artists blatantly absent from the present collection.) The prodigious Cuevas of Mexico, who received a major retrospective at this museum in 1978, is well-represent here.

On the way up the broad, winding stair are the first of several large, abstract paintings called "Nipo-Brazilian," the work of first and second generation Brazilians of Japanese ancestry. There are, says Sarda, more Japanese living in Sao Paolo than in any other city outside Japan. A lot of them seem to be first-rate painters.

On the second floor, two new installations have been arranged, the first being a room full of captivating "primitives" by artists from Jamaica, Panama and Haiti. A crucifixion by Haitian sculptor Georges Liautaud is matches. The new installation of works on paper, previously in storage, represents a wide-ranging group of contemporary drawings, watercolors and prints, accented with sculpture by several good artists, including two from Washington -- Rudy Ayoroa and Alfredo Halegua.

A final gallery features the most contemporary works, among them a kinetic wall-piece by Jesus Soto (who had a retrospective at the Hirshhorn a few years back), and Alejandro Otero, also of Venezuela, whose popular wind-sculpture stands next to the Air and Space Museum. An extraordinary female torso covered with leather by Beatriz Echeverri of Colmbia is one of many works by women artists in the collection. A bold abstract painting by Angel Hurtado reveals another facet of this artist, who has produced documentary films on 15 of the artists in the Museum's collection. The films can be seen, free of charge, by prior arrangement. Special tours are also willingly arranged.

The gallery is open Monday through Friday, 10 to 5. Admission is free.