Medical experts said yesterday that it is too early to speculate on the exact nature of President Neil L. Rudenstine's medical condition.

Dean of the Medical School Daniel C. Tosteson '46, Rudenstine's medical spokesperson, said in a statement that the president "is suffering from severe fatigue and exhaustion of unknown origin."

"Diagnostic studies on an outpatient basis are in progress," Tosteson said in the statement.

Professor of Medicine Dr. Frederick P. Li said fatigue and exhaustion are very general symptoms. The president could have anything, he said.

But two Medical School doctors said it is unusual for someone with fatigue to take a medical leave, as Rudenstine has done.

Usually, people with demanding schedules who are suffering from fatigue will take a few weeks of vacation, the doctors said.

Harvard officials emphasized yesterday that Rudenstine's leave would not be a vacation. He is scheduled to undergo continued medical tests, and his exact diagnosis is uncertain.

A physician at Massachusetts General Hospital, who spoke on condition of anonymity, said Rudenstine could have anything from easily treatable fatigue to a serious medical illness.

One reason why the president's diagnosis is uncertain could be that there is a long list of diseases for which fatigue is a symptom.

"These symptoms could be common in any age group," the physician said. "The list of conditions that fit the symptoms of exhaustion and fatigue is a mile long."

Beth Huneycutt, director of information services for the Chronic Fatigue and Immune Dysfunction Syndrome Association of America, said that approximately 24 percent of Americans report symptoms of severe fatigue, like Rudenstine's--though not all are the serious chronic fatigue syndrome.

Dizziness, irritability and confusion are common symptoms of chronic fatigue.

"Chronic fatigue is a very highly prevalent complaint heard by general practitioners," she said.

While doctors wait to make a more precise diagnosis, "severe fatigue of unknown origin is sometimes something that can be given as an intermittent diagnosis," Huneycutt said.

There are several possible causes for severe fatigue, she said. "There's not really a certain level of sleeplessness. Diet factors into it. It's very individualistic."

Alan Bell, co-founder of the Foundation for Environmental Health Research in Arizona and an expert in chronic fatigue syndrome, said the causes of fatigue are not fully understood.

Symptoms of chronic fatigue syndrome include muscle weakness, headaches and dizziness, Bell said.

Jonathan A. Lewin and Anna D. Wilde contributed to the reporting of this story.