The folks at the Hirshhorn Museum and Sculpture Garden aren't really black-tie kind of people. The art openings there have been lively but decidedly casual. The last formal party anyone can remember was for Joe Hirshhorn's 80th birthday in 1979.

"I never had a tuxedo until I left Des Moines 15 years ago," said Director James Demetrion before last night's gala exception to the museum's informal rule. His tux was a going-away present from his Iowa pals who wanted him to be properly attired for fancy Washington events. Problem is, Demetrion loathes black-tie affairs--and successfully avoided donning his penguin suit until last night. "Of course I've outgrown it, but that's not going to keep me from wearing it," he said with a wry smile.

Only a very special occasion would have forced his hand: the Hirshhorn's 25th-anniversary gala, "A Celebration of Art," a $1,000-a-plate fund-raiser for the only Smithsonian museum of modern art. The Hirshhorn is neither the oldest nor largest institution devoted to such works, said Demetrion, "but no one surpasses our museum in our love, passion and respect for modern art."

The feeling is mutual, judging by the artists who showed up for the bash. The guest list included some of the art world's most illustrious names: Robert Rauschenberg, Christo, George Segal, Jeff Koons, Brice Marden and Mark di Suvero, as well as collectors, dealers and fans of smart, challenging paintings, photographs, videos and sculpture that defy conventional description.

"It's Washington's museum of modern art," said figurative artist Joe Shannon. "It deals with the unfolding edge: very advanced, sometimes very experimental, sometimes very repugnant."

That edge was on display as more than 500 people opened the party with cocktails and a viewing of the anniversary exhibit, "Regarding Beauty: A View of the Late 20th Century." The show is provocative, intriguing, sometimes perplexing but never dull.

"What's a self-lubricating plastic frame?" asked one guest, staring at the description accompanying the picture of a woman wearing a garter belt and shoes with suction cups. Not an Old Master, to be sure, but nothing to send bluenoses screaming for the exits either.

The Hirshhorn, said photographer Bill Christenberry, manages the delicate act of balancing the intellectual with the new: "They do strong and even tough things, but it never goes into the arena of trying to attract sensationalism."

"It's somewhat changed by the radical new art," said collector John Hechinger, raising one of his very distinctive eyebrows. "You have to consider the Hirshhorn on the frontier. They've gone considerably beyond it." He paused diplomatically. "That's not a criticism but a critique."

"If Giuliani saw this, he'd try to shut this down, too," Washington lawyer Lloyd Cutler said with a laugh, speaking of New York's mayor.

Given the conservative nature of the nation's capital, it's remarkable that a federally funded museum has managed to display cutting-edge art and yet avoid the heated debate most recently centered on the controversial Brooklyn Museum of Art show.

"Museums take modern art very seriously," said Neal Benezra, the Hirshhorn's assistant director. "It's a shame when it becomes a political issue."

But contemporary art is always risky. The artists are still alive, still creating new pieces, still experimenting. The art is often difficult to understand and appreciate. "Keep in mind the impressionists," said Benezra. "People were appalled by that. It takes a while to get used to new work."

Last night's gala raised $100,000, which will be dedicated to exhibitions, programs and education. The Hirshhorn is blessed with a fat acquisitions fund, thanks to private trust money, which adds to the museum's very strong beginnings. The gallery opened in 1974 with 12,000 objects donated by its founder, Joseph Hirshhorn (1899-1981), and has carefully built a world class collection over the past 25 years.

"The Hirshhorn's focus is on art in a way that I think is quite singular in Washington; it's about keeping the best of the old while celebrating the new," said Robert Lehrman, chairman of the museum's board and a contemporary art collector. "This gala is a way of showing respect and gratitude for the best of the public-private partnership that was initiated by Joe Hirshhorn's generosity and Congress's commitment to living artists."

The living artists on hand looked suitably artsy. Rauschenberg showed up in a silver shirt and no tie; Marden choose a black jeans-and-jacket ensemble. "If you're going to wear jeans, you might as well wear expensive shoes," he explained.

Christo, also sans tux, said his admiration for the Hirshhorn is great. "Mr. Hirshhorn was collecting for personal, private tastes," he said. "Art always involves passion. It cannot be created by bureaucratic committees. It's a personal adventure."

But his respect will not extend to covering the building in one of his famous wraps to celebrate its anniversary. "We did already a museum," the French artist shrugged.

But some grand celebration was clearly in order. First lady Hillary Rodham Clinton hosted a tea Tuesday afternoon in the White House sculpture garden, heaping praise on the Hirshhorn's founder, its directors and the vision represented by the power of modern art. "This gift has stood the test of time," she said.

Last night's dinner was served under a clear tent surrounding the museum's courtyard fountain. The tables were decorated in rich color blocks to resemble a shimmering abstract painting. There were gazing balls on the tables and in the fountain to reflect the lights and images of the night.

It was beautiful, dramatic and just a little edgy.

Olga Hirshhorn, the founder's widow, looked pleased. "This is very exciting," she said. "Joe would be very happy."

CAPTION: Hirshhorn Museum Director James Demetrion, center, and wife Barbara chat with St. Louis art collector Barney Ebsworth.