When Joe Hirshhorn first met Larry Rivers, the Bronx-born artist was just emerging as an enfant terrible in the explosive New York art world of the '50s. A jazz musician and realist painter, Rivers was attempting to buck the onrushing tide of abstract expressionism, and he wasn't doing badly. In 1949 critic Clement Greenberg had called him "a better composer of pictures than was Bonnard," a nice start for a 26-year-old saxophone player.

Hirshhorn recalls their first meeting in the hallway of an office building he shared with Rivers' dealer: "The kid knew I was a collector, and asked me to come see his stuff. I liked it." He bought four paintings that day.

Hirshhorn's enthusiasm continued and proved to be important, for as a result the Hirshhorn Museum now owns 45 of Rivers' paintings, including some of his best.Just in time for the artist's 60th birthday next month, they all go on display today for the first time. The show helps to bring into focus this irreverent, poetic and highly individualistic painter. As always with such shows, it leaves us wanting to know more.

Back in the '50s, when the first generation of abstract expressionists were passionately slashing away, Rivers was an intrasigent realist, carrying out his revolution chiefly in terms of the subjects he chose to paint: his elderly mother-in-law stark naked, for example -- a shocker at the time. In 1955, he created an even greater stir when he showed his startling take-off on one of the great icons -- and cliches -- in American art: Emanuel Leutze's gigantic, academic "Washington Crossing the Delaware." The Museum of Modern Art brought the painting and showed it, setting off sparks in imaginations all over town. It became a clear-cut precursor not only of pop act, but of a sub-genre of pop which devoted itself chiefly to doing takeoffs of famous works of art, such as Warhol's "Mona Lisa" and Lichtenstein's "Picassos."

Rivers, too, continued to make paintings which had other art -- usually older art -- as their source, but not so much for the purpose of doing take-offs as for reinterpreting the images. His were loosely painted, free-hand, blurred multiple images with an unfinished look, which became his characteristic style. He copied a Rembrandt, for example, from the lid of a cigar box upon which it was emblazoned. He made a free-hand, painterly rendition of the Louvre's "Mademoiselle Riviere," calling it "I like Ingress, Too," lest anyone think he was only interested in cigar-box versions of the Old Masters.

Among the best known of these paintings is the Hirshhorns' "The Greatest Homosexual," based on the 1812 Jacques-Louis David portrait of Napoleon in the National Gallery -- now his unlikely neighbor across the Mall. As Rivers explained of this double-imaged painting, filled with areas of erasures and collage, this was in the tradition of a contemporary artist "paying homage" to a brilliant ancestor.

"But after many days of drawing, brushing, cutting, gluing, stencilling, etc.," said Rivers, "I couldn't avoid some obvious nontechnical conclusions. Given a right hand nesting in the split of the cream vest, a gesture which by itself has come to represent Napoleon, the plump torso settled comfortably on the left hip, the careful curls and coif, the cliche of pursed lips -- I decided Napoleon was gay. Now if he wasn't history's 'Greatest Homosexual,' who was -- Michelangelo?"

There are other lesser known surprises here -- small ones, such as the splendid drawing of his friend Willem de Kooning, and giant ones such as the billboard he painted high up over Broadway to announce the first New York Film Festival in 1963. In 1965 he produced another giant work, a mural-scaled assemblage of small paintings and three-dimensional elements consciously created to be a masterpiece, which it may well be. "The History of the Russian Revolution from Marx to Myakovsky" is an epic visual narrative that relates the events of the Russian Revolution, starting with the theorists and ending with a poignant poem of disillusionment by the poet Mayakovsky, who committed suicide. This work, a permanent installation at the Hirshhorn, takes on new poignancy and meaning in this context as a summing up of Rivers' oeuvre.

But Rivers, at 60, is still abloom with new ideas and new energy, exemplified here in a brand-new drawing on giant scale, entitled "Chinese Information -- Travel," a composite of two great scroll paintings that Rivers found in a book on Chinese art. Hirshhorn saw the drawing last year, brought it on the spot, and gave it to the museum. It is being shown here for the first time. For viewers that develop an appetite for a Rivers of their own, the artist has created a poster for this show which will be available through the Smithsonian Associates. The show continues through Sept. 20.