Alzheimer's begins to show itself by causing relatively subtle changes in memory, judgment and reasoning. People with the disease can then go on to have difficulty remembering what they said or read a few minutes earlier; they forget the names of relatives and friends. Later, they may have difficulty completing simple tasks.

So in looking for early signs of mental impairment, experts say, doctors generally ask about and observe the way a patient transacts the routine business of the day. What makes this early diagnosis so hard is that these are the same things people tend to forget as they age; unlike, say, breaking a bone, the onset of Alzheimer's cannot be pinpointed. And while certain tests can strongly indicate Alzheimer's, there are no specific blood or other tests to confirm the diagnosis while the patient is alive.

Still, his occasional lapses notwithstanding, the doctors said they had seen no significant changes in Mr. Reagan's mental competence in the White House. From his election in 1980 until he left in January of 1989, they said, the President was always well clear of that fuzzy line where forgetting becomes Alzheimer's.

He ''never forgot appointments, misplaced or lost things, where he put his glasses, never forgot to put his hearing aids in, never forgot to put his contact lenses in, and these are things he did for himself,'' Dr. Mohr said. ''I saw him saddle and bridle horses at the ranch and later put things back exactly where they belonged.'' And Mr. Reagan, the doctors stressed, was punctual, never depressed and had no difficulty with language or understanding what was going on around him.

Although White House doctors seldom join working sessions, they stay nearby from the time the President goes to work until he retires for the night. That intimate relationship, Dr. Mohr said, ''allowed us to interact with our patient, Mr. Reagan, in a way that most physicians do not interact with their patients.''

Their closest observation came when Mr. Reagan spent weekends at Camp David or vacationed at his California ranch, where, in addition to Secret Service agents, a doctor and a military aide were usually the only White House staff members present. Before dinner, Dr. Hutton said, the President usually spent an hour talking, swapping stories and reciting the poetry of the Klondike balladeer Robert Service. In those sessions, Mr. Reagan had a keen memory both for contemporary affairs and past events, like games he had described as a sports announcer.

Drs. Hutton and Mohr, along with Dr. Daniel A. Ruge, a neurosurgeon who was Mr. Reagan's physician in his first term, said they had also evaluated his mental status by asking him to subtract 7 continually, starting with 100, and by asking other standard questions in annual check-ups. But the fourth doctor, T. Burton Smith, a urologist who was the chief White House physician from January 1985 to January 1987, said he been less thorough at times in examining neurological function.