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.

Dean of the School of Public Health Harvey V. Fineberg '67 said in a statement that the deans of the University's various faculties had met with Rudenstine yesterday. Fineberg's statement said the president would appreciate notes of "support and encouragement."

"While he has been burdened by a lack of sufficient sleep over a period of months, his spirits are very good, and he is looking forward to restored vigor and to returning to his post in a matter of weeks or months," Fineberg said.

"I know that the president acceded to this decision very reluctantly, a conclusion that was forced upon him by circumstances and his doctor's firm instructions," Fineberg added.

Carnesale is already serving as both provost and dean of the Kennedy School of Government. It is not clear what will happen to either of those posts.

But Peter L. Malkin '55 said a situation such as this "was something that people were considering when the provost was created."

Carnesale said at an interview yesterday that the new Kennedy School dean could be chosen in the next month.

"I certainly wouldn't rule out something by Christmas," the acting president said. "The search will continue at a pace. This is a strong argument for not delaying it."

Charles Slichter '45, the senior member of the Corporation, said in a statement that he is eager for Rudenstine's return.

"Neil Rudenstine has been doing a remarkable job of meeting the extraordinary demands of the Harvard presidency," Slichter said. "The entire Harvard community wishes him a speedy and complete recovery. We are eager to have him back with us."

"With Provost Carnesale serving as Acting President, we know that the University will continue to have the benefit of strong leadership in the interim," Slichter added.

Rudenstine did not attend the press conference. The president was at home resting, Wrinn said.

Sources close to the president said that although Rudenstine has looked worn and tired in the recent weeks, there was little warning of the leave. In the week before Thanksgiving, the president gave five speeches in three days.

Some sources also said that this is not the first time Rudenstine has taken time off for exhaustion. He took a few days off in February for the same reason and took a similar leave at Princeton.

The president is known to have chronic back problems, but there was no indication that they contributed to his doctors' decision to have Rudenstine a leave.

"He has had a back condition that over the years has given him obvious pain," said Robert Hollander, a Princeton professor who knew Rudenstine "fairly well" during his tenure there.

But Hollander said that to his knowledge, Rudenstine has never before had to leave work because of severe exhaustion.

Much of Rudenstine's time in the last several months has been spent fundraising for the University's $2.1 billion capital campaign.

"The effect on campaign depends on how long he's out," Daniel said. "If it's a short time period, I don't think it will be [affected], at least we certainly hope not."

Carnesale will take Rudenstine's place at a meeting of Ivy League presidents today. He will also attend this weekend's Board of Overseers meeting in place of Rudenstine.

"I will perform the responsibilities of the office of the president," Carnesale said. "This is not a waiting period. We will continue on course."

"We'll miss his leadership for this period," Carnesale added. "I expect to be talking with him, though the frequency of the conversations will be determined more by his physicians than by me."

Rudenstine is scheduled to make a decision concerning the future of Harvard's relationship with ROTC. He was to make a recommendation to the Faculty Council in December.

Carnesale said that Rudenstine's report is "just finished" and will be distributed to the Faculty Council before their next meeting.

"His recommendation will be contained in a document... to be distributed," Carnesale said in an interview after the press conference. "We don't have to do artificial things about presenting it."

The president was also planning a fundraising trip to Latin America in January.

"I don't think any actions like [canceling the trip] have been taken," Carnesale said.

The only light moment of the press conference was when a reporter asked about the line of succession for the president's job.

"I can't tell you about the line of succession," Carnesale joked, "but the Speaker of the House is in there somewhere."

A cursory search of presidential files in the Harvard University archives today found no examples of presidents taking leaves because of illness.

Carnesale said at the press conference that President Derek C. Bok took a sabbatical in 1987, during which former-Dean of the Faculty Henry Rosovsky assumed the role of acting president.

"There are precedents for the president being on leave before," Carnesale said.

Several Meetings

There were "several meetings" of University officials over the Thanksgiving weekend, Wrinn said.

The Corporation met this weekend, according to University Treasurer D. Ronald Daniel.

"I'm disappointed but [Rudenstine] was exhausted and will have some tests," Daniel said. "Carnesale will be fine. He will pick up a lot of the responsibilities, but we will obviously Miss Neil. He was particularly effective."

University officials expressed little surprise at Rudenstine's exhaustion.

Former President Nathan M. Pusey '28 said this afternoon that he did not know about the move, but said he was afraid something of the sort could happen.

"I've been worried about President Rudenstine because he has been working so hard," Pusey said. "He's trying to do everything at once."

"He's a man who works very hard and needs a vacation," Allen Proctor, the new finance vice president, said. "It should renew him. We all need one. Presidents just don't get them very often."

But Daniel cautioned that Rudenstine's leave is "definitely not a vacation. It is a medical leave of absence."

Former Provost Jerry R. Green, Leverett professor of political economy, also said yesterday that he had not heard anything about Rudenstine's leave of absence before the announcement.

"I was surprised," said Green, who added that he had not seen the president for a while. "I wish him a speedy recovery."

Jonathan N. Axelrod, Elizabeth T. Bangs, Gaston de los Reyes, Stephen A. Engel, Stephen E. Frank, Marion B. Gammill, Jonathan A. Lewin, Sarah E. Scrogin and Andrew L. Wright contributed to the reporting of this story.Crimson File PhotoNEIL L. RUDENSTINE