The Hirshhorn Museum and Sculpture Garden's Assistant Director Neal Benezra surprised museum trustees and staff yesterday by announcing he will leave Washington at the end of the year for the Art Institute of Chicago, where he will be deputy director as well as head of the department of 20th-century painting and sculpture.

He'll be second in command of one of the country's greatest and largest art museums, with collections that range from ancient Chinese bronzes and Greek vases to French impressionists and contemporary art. Its annual operating budget of roughly $75 million (not including its renowned art school) is nearly 20 times that of the Hirshhorn.

His departure comes as a blow to the Hirshhorn, where he seemed the most likely successor to Director James Demetrion, 69.

As he headed into a meeting to inform the staff yesterday morning, Benezra said his new position is "an embarrassment of riches."

"I have a wonderful position here, and have had an enormously exciting time here working with Jim, who's been one of the handful of really important people in my life," he said. "But I've always been a big believer in the fact that individuals need to challenge themselves. And the position at Chicago--previously held by two senior people--is compelling, exciting and challenging.

"It's also a city that my wife and I loved living in for six years--a great museum in a great town." Benezra's wife, Maria Makela, an art historian teaching at the Maryland Institute College of Art, has published extensively on 20th-century German art. "Frankly, we're looking forward to her being able to spend more time with our daughter, who is 6," he said.

Benezra, 46, was a contemporary curator at the Art Institute before coming to the Hirshhorn as chief curator in 1991. He soon rose to his present position of assistant director for art and public programs. But administration never bogged him down: He rehung all the Hirshhorn's galleries, making the permanent collections far more logical. He organized several landmark shows including "Bruce Nauman," "Stephan Balkenhol," "Distemper: Dissonant Themes in the Art of the 1990s" and the present 25th-anniversary show "Regarding Beauty: A View of the Late 20th Century," which opened to critical acclaim last week. He also wrote or co-wrote several major catalogues. He's been active in introducing young European artists to the collection.

In Chicago, Benezra will also wear two hats. As deputy director, he will oversee the functions of museum education, the libraries, publications, graphic design, imaging (photographic and audiovisual services and new technologies) and art conservation. And as Frances and Thomas Dittmer curator of 20th-century painting and sculpture, he will oversee that department's large collections, exhibitions and staff. He said he will complete three shows now in the works at the Hirshhorn, including an Ed Ruscha retrospective due to open next year.

"I've got mixed feelings," Demetrion said. "You've got to congratulate the Art Institute, and it's great for the Midwest. But for us it will be a major loss. Life will go on, but it won't be the same.

"I hold him in the highest regard," he added. "He's a terrific curator and jargonless writer and a great human being as well. You can get some real jerks who are wonderful curators, but they're impossible to work with. He's going to be very hard to replace. You can add as many 'verys' as you want."

CAPTION: Neal Benezra, leaving the Hirshhorn for the Art Institute of Chicago.