Hirshhorn Director James T. Demetrion, who led the 27-year-old museum for nearly 17 years, will retire in September.

No successor has been named. The Hirshhorn Museum and Sculpture Garden, on the National Mall, is the Smithsonian's showplace for modern and contemporary art. It drew 900,000 visitors last year.

Demetrion told his staff of his decision yesterday.

"It's been in the back of my mind for a long time now," Demetrion said in an interview yesterday. "A museum director can't go on forever. I'll be 70 in July. And I wanted to get the Clyfford Still show over with. So it seemed a logical time."

The Still exhibition, opening June 21 and running through Sept. 16, will explore that artist's unexamined contribution to abstract expressionism. It will be the fourth show Demetrion has organized for the museum, following retrospectives of Francis Bacon, Jean Dubuffet and Stanley Spencer. He is also credited with bringing several exhibitions by important European figurative artists to Washington, including a Lucian Freud show in 1987. In the early '90s, he also initiated the redesign and re-landscaping of the museum's once bleak and inhospitable plaza, greatly enhancing it as a setting, both for visitors and for outdoor sculpture.

As the museum's second director, Demetrion has been credited with transforming what was Joseph Hirshhorn's vast and quirky private collection into a world-class museum of modern and contemporary art.

"A museum that begins with a private collection and doesn't continue to collect becomes a tombstone," he said back in 1985. At that time, the museum had little or no recent work and collecting "the art of our time" became his top priority.

The problem, however, was that when Demetrion arrived in Washington, the acquisitions budget was a paltry $150,000 a year -- 50 percent less than he'd had to spend as director of the Des Moines Art Center, his previous post. He met the challenge by carefully selling hundreds of works into the red-hot art market of the late '80s, winnowing out redundancies and lesser examples. Combined with prudent management, ingenuity, fundraising and good luck, he built the acquisitions endowment to $25 million within a decade. He hates talking about money, but in fact one of Demetrion's greatest legacies will be the acquisitions fund he leaves behind for his successor. It is now well over $30 million. "It was all made possible by the generous terms of Joe Hirshhorn's gift, which stated that any of the 12,000 items in the collection could be deaccessioned, or sold, as long as the proceeds were used to buy more art," said Demetrion. "That's been the central factor in building this endowment."

Mostly, Demetrion said, "I've enjoyed working on the collection. That's been the most satisfying part, and gave me the greatest pleasure."

"He's going to be difficult to replace," said Tom Lentz, director of International Art Museums of the Smithsonian. "Jim has always managed to remain a real human being in a very competitive and high-powered field. But ultimately, it's all about the art for Jim. He's redirected and refocused that collection and has put the museum on a very strong path in terms of contemporary art.

"He's so modest and self-deprecating about his power and accomplishments. But under it all is a fine mind and a superb eye. He's going to leave his mark."

The search for a replacement will get underway quickly, Lentz said. "We'll cast our nets pretty wide. It's clear we won't get another Jim Demetrion, but we want someone who can build on his legacy, and move the museum in other directions as well."

Demetrion isn't sure what he and his wife will do next: "I don't know if we'll stay here or move out west," he said. "First I have to straighten up my room, which will take two years. And drive my wife crazy." Hirshhorn Museum Director James Demetrion will retire in September.