The Hirshhorn's new Frank Stella is a knockout. "Quaqua ! Attaccati La ! 4X," a 13 1/2-foot-high assemblage of 1985, is among the strongest pieces of contemporary art to find a home in Washington in the past half-dozen years. It is enormous, and enormously impressive. Its paint had barely dried when it was picked, from Stella's studio, by James T. Demetrion, the museum's new director. It was purchased some months later. The buy was not announced.

If you think you know the Hirshhorn's permanent collection, it is time to look again.

Other recent acquisitions, all of them unpublicized, have been placed on public view there. Hanging on the second floor is "The Hunt of the Two-Horned Creatures" (1963), an exceptionally fine oil by the late Jean Dubuffet. A terrific steel horse made in 1985 by America's Deborah Butterfield dominates the lobby. "Inside Ohio Bones" (also 1985), a painting by Robert Stackhouse, who used to live in Washington and still teaches at the Corcoran, is displayed a floor below.

The new objects on display are just the iceberg's tip. There are changes going on at the round museum on the Mall.

The collection has been growing at an extraordinary rate, and not through Jim Demetrion's purchases alone. On the top floor of the building, behind white partition walls, curators are processing Joseph H. Hirshhorn's last bequest. When the museum opened in 1974, it owned about 6,000 works from his vast collection. In later years he gave more than 1,000 others. Hirshhorn died in August 1981, and his bequest is here at last. It includes 1,666 paintings, 876 sculptures, 1,189 drawings, 96 collages, 1,356 prints, 116 pieces of decorative art and a small group of photographs -- about 5,300 works in all. Selections from the bequest will go on view in August.

The collection, although growing, is also being pruned -- through auctions and dispersals. Though Hirshhorn best loved modern art, he bought other things as well, Eskimo, South Arabian and pre-Columbian artifacts, Benin bronzes and the like. About 700 objects from his initial gift have lately been distributed to various other branches of the Smithsonian Institution.

And the Hirshhorn's personality is being changed as well. It's been only 18 months since Demetrion took over, but already the museum has begun to bear his stamp.

It shows in small but telling ways in his installations. He has filled a little gallery with five haunting works by Balthus. He's arranged an instructive confrontation between the Bay Area figurative painters of the 1950s (David Park, Elmer Bischoff, Nathan Oliveira and Richard Diebenkorn) and England's David Hockney, who went to the West to find the sun in 1964. He has even changed the founder's bust that greets one at the door. The bronze that stood there once was stately and old-fashioned. The 1967 plaster head (by Pablo Serrano) that has since replaced it is jauntier by far. It shows Joe Hirshhorn as he was.

These are subtle changes. More instructive are the ways in which Demetrion has been adding to -- as well as subtracting from -- the art the Hirshhorn owns.

He says, "I didn't know Joe Hirshhorn, but I have to assume that he bought, at least at first, largely for himself." Hirshhorn liked to fill his homes with lots of little objects. And when he came upon an artist he admired, he happily, voraciously, bought that artist's work in depth. The breadth of his collection, much maligned at first, seems to me its greatest strength.

Demetrion, however, has picked another path. He seeks high-impact objects. Large groups of worthy, lesser things do not much attract him. In updating the collection, Demetrion intends to buy only those key objects that he believes deserve inclusion in the U.S. government's museum of modern art.

He's bought a room-sized assemblage by Ed and Nancy Kienholz, a Robert Irwin disk of 1969 and a soft sculpture by Claes Oldenburg ("they're hard to find," he says; the one he has selected, "Soft Engine for Airflow, with Fan and Transmission," was made in 1966). He's bought an Isamu Noguchi bronze, "Cronos" (1947), and an early Diebenkorn, "Berkeley Number 22," an abstract oil done in 1954 just before the painter took his brief but famous venture into figuration.

Because the verdict is already in on Diebenkorn, Noguchi, Oldenburg and Irwin, purchasing their early work is relatively risk-free. But Demetrion's not timid. He has bought much that is brand-new.

Following his lead, the Hirshhorn's board has purchased a 12-foot-wide canvas by northern California's William Wiley, and an even larger picture in oil paint and lead by Germany's Anselm Kiefer. Both were made last year. The wooden grid by Sol LeWitt that was bought at his suggestion is dated 1984. A show of recent acquisitions is scheduled for November.

"I've pretty much made up my mind," Demetrion says, "though I'm not set in concrete, that I'm not going to buy anything made prior to World War II."

Hirshhorn and Abram Lerner, his longtime friend and colleague and the museum's first director, saw eye to eye on many things. Both men were New Yorkers, both remembered the Depression, and the spirit of their city, and the spirit of their time, shows in what they bought.

But Demetrion is a midwesterner of a younger generation. He is a burly man, now 55. There is no trace of eastern chic in his clothing or his speech. He was born in Middletown, Ohio. Walter Hopps, a former director of the Corcoran who now runs the Menil Collection in Houston, gave Demetrion his first museum job. In the early 1960s they worked together in California at the Pasadena Art Museum, where Demetrion eventually replaced him as director. Demetrion moved to Iowa to run the Des Moines Art Center in 1969.

In selecting for the Hirshhorn, he's picked California artists (Diebenkorn, Irwin, Wiley, Edward Kienholz, Robert Arneson and John Altoon among them), and artists from Chicago (H.C. Westermann, Leon Golub), Germany and Spain.

And while he has been acquiring, he's been selling, too.

Fifteen works of art from the Hirshhorn's permanent collection have been sold so far this month at Christie's in New York. Four works on paper by Franz Kline and an oil by Hans Hofmann were auctioned off two weeks ago. The Hofmann fetched $30,000, the Klines $51,300.

On Thursday the museum sold a small bronze by Degas. It brought $32,500. A bronze head by Matisse did far better than expected: The presale estimate was $150,000 to $200,000, but it sold for $396,000. "Seated Woman" a 1956-57 bronze by Henry Moore, also did extremely well. It sold for $990,000, far above the estimated $550,000 to $750,000. All these hammer prices include a 10 percent commission.

On Friday the Hirshhorn auctioned off a late statuette by Renoir and five works by Marino Marini, three drawings and two bronzes. All these pieces, too, did better than expected. The Renoir (presale estimate: $20,000-$30,000) was knocked down for $48,000. The five Marinis (aggregate presale estimate: $77,000-$106,000) sold for a total of $150,700.

Twenty-four other objects -- among them bronzes by Degas, Daumier, Picasso, Reuben Nakian and Jacob Epstein, and pictures by Childe Hassam, George Inness and Winslow Homer -- will be sold later this year.

Though some portion of the take may go into the endowment, all the money raised eventually will be spent on museum acquisitions.

Joe Hirshhorn was insatiable. These sales tell us much about the depth of his collection. They hardly dent his gift.

The $990,000 Henry Moore had been displayed outside for years, first at Hirshhorn's home in Greenwich, Conn., and later on the Mall. The museum owns another cast of the same sculpture -- one in far finer condition. The Degas bronze sold is also a duplicate of a cast in the collection, as is the Daumier, as is the Matisse. Although four works by Kline were sold, the museum retains nine. One Hofmann went to Christie's, but nine others are still here. Five Marinis were sold. Twenty-two remain.

"We have a couple of ground rules for deaccessioning," says Demetrion. "We don't dispense with anything by a living American, unless we have that artist's permission. For instance, we are going to sell Reuben Nakian's 'Head of Marcel Duchamp.' We have another cast of the same piece, but we still asked him first. And if Joe Hirshhorn and Al Lerner bought something by an artist, we won't exclude that artist. We won't sell anything that artist made unless other works remain.

"Although it bears his name, this is not a memorial museum," says Demetrion. "Joe agreed to that."

Hirshhorn loved a bargain. Demetrion is careful with his money, too. One impressive fact about his recent purchases is how little he has spent.

Though, since his arrival, the museum's board has purchased more than 20 objects, all of them significant, in total they have cost less than $1.5 million.

The giant Stella sold for less than $150,000. The enormous Kiefer cost less than that. Older works acquired have cost considerably more, but none has cost as much as $500,000.

Last December the National Gallery of Art spent more than $4 million on a single portrait by Rembrandt Peale. Earlier this month a single work by Jasper Johns, one of his first "Targets," was "bought in" at public auction, even though the bidding had reached $2 million.

"In 1971, when I was still in Iowa, we bought 'Tennyson,' a 1958 work by Jasper Johns," says Demetrion. "It was the first Johns sold to a museum outside of New York, and it cost a lot, or what then seemed a lot -- $88,000. I remember that a trustee asked me at the time, 'Wouldn't it be better to buy 22 less expensive works at $4,000 each?'

"Prices for the best things rise. Take that Johns 'Target' that just failed to sell for $2 million. When I was in Pasadena, Walter brought it in, hoping to find a buyer. He couldn't. The selling price in those days was just $8,000."

The Hirshhorn is not rich. Demetrion is intent on building up a war chest for buying works of art.

"When I came to the museum, we had about $1.3 million set aside for purchases," says Demetrion. "Later we got more. When we transferred what we call the 'special collections' -- those African and Eskimo and pre-Columbian things that Hirshhorn had collected -- we made an arrangement with the Smithsonian. I wouldn't call it a sale. But the deals were connected."

Though Hirshhorn early on had provided $1 million with which to purchase art, that money was instead spent on the Hirshhorn's building. When the transfers were arranged, the Smithsonian's board of regents agreed to replace that lost acquisitions fund. The regents figured that the $1 million, properly invested, by now would have increased to $2.5 million, so they came up with that much. It's gone into the endowment. They also provided $700,000 in place of the interest Hirshhorn's money would have earned.

Demetrion thus had at his disposal something more than $2 million. That includes the initial $1.3 million, plus that $700,000 interest, plus the $150,000 that Congress gives the Hirshhorn for purchases each year. Before last week's successful auctions, he had approximately $600,000 left.

"That $150,000 from the government is not a lot," he says. "The Tate in London gets an annual acquisition budget of $2 million. The Centre Pompidou in Paris gets $3 million. There are holes in our collection. I haven't made a list, but I know that they are there. We'd love a major Ensor, or a major Klee. We'd love a major Jackson Pollock, but who wouldn't? We do the best we can."