In an unexpected move, the University announced yesterday that Neil L. Rudenstine, the 26th president of Harvard, will take a medical leave of absence for at least several weeks.

Dean of the Medical School Daniel C. Tosteson '46, who is serving as medical spokesperson for Rudenstine, 59, released a statement yesterday afternoon which University officials called the full extent of information on the president's condition.

"He is suffering from severe fatigue and exhaustion of unknown origin," Tosteson's statement said. "When the results of these studies are available, a more certain prognosis will be possible. It is likely that the president will be on leave for a matter of weeks or longer."

Albert Carnesale, the University's provost since July 1, has become acting president.

Carnesale spoke at a terse and tense 15-minute press conference in the reading room of the posh Faculty Club. Reporters from newspapers and wire services across the country attended, and at least 10 television cameras were in the audience.

Carnesale repeatedly refused to identify Rudenstine's physicians. He referred all medical questions to Tosteson.

During a 15-minute press conference, Carnesale said Rudenstine told him last Wednesday of his decision to take a leave. The provost said he had "no inkling" of any medical problem when he assumed his post July 1.

"I'll be seeking some interim help with the responsibilities [of provost]," Carnesale said.

"He's not bed-ridden," Carnesale continued. "He's not hooked up to anything. What he is tired."

Carnesale said the president had been told by his doctors to take a leave of absence.

"It's as close as one can get to an order from his physicians," he said.

The Harvard Corporation, the University's chief governing body, confirmed the announcement in a statement released this afternoon.

"[Rudenstine] is suffering from severe fatigue and exhaustion. Diagnostic studies are in progress," a statement released by the Harvard Corporation, the University's chief governing body, said. "In his absence, the Provost of the University, Albert Carnesale, will serve as Acting President."

The president will be gone for "weeks or months," said Joe Wrinn, director of the news office.

Rudenstine said in a statement that he decided to take a leave "with the greatest reluctance."

"My clear desire would have been to continue to work uninterruptedly with my colleagues at Harvard," the president's statement said.