Nearly three years after abruptly resigning as prime minister of Israel, Menachem Begin remains withdrawn and distant in the privacy of a small apartment overlooking the spectacular Jerusalem Forest, maintaining an uncharacteristic silence that baffles his closest friends.

Clad most of the time in nightclothes and a robe, the once-fiery don of Israel's often-contentious parliament is strangely subdued in his self-imposed isolation, according to his closest friends and confidants.

He reads prodigiously and listens to news broadcasts on Israeli radio and television and the British Broadcasting Corp. with an almost slavish regularity, they said.

But his absorption of current events appears to be a one-way street: Save for brief, obligatory letters he dictates to a secretary in response to the steady flow of mail he still receives, Begin has maintained a sphinx-like silence about the tumultuous events of 1983, when he stunned his constituency by resigning amid the trauma of the war in Lebanon, telling his Cabinet ministers, "I cannot go on."

Begin, now 73 and in failing health, has been outside the walls of his apartment publicly only four times since then: to move from the prime minister's residence three months after leaving office, to visit his wife's grave on the Mount of Olives twice and to enter and leave the hospital in September 1984 for a prostate operation.

Begin's last published photograph was taken nine months ago, when he went to visit the grave of Aliza, his wife of 37 years, and appeared in the next morning's newspapers as a drawn and haggard shadow of the former Jewish underground guerrilla leader who dominated Israeli politics from 1977 to 1983.

However, Begin's closest confidants and most frequent visitors say that the former prime minister in recent months has regained weight and is as mentally alert as ever, despite persistent reports in the Israeli press that he is seriously ill.

"I wish that I would look so good. He is still strong, but he doesn't want to be involved in anything," said Yehiel Kadishai, Begin's personal secretary, who visits him for two hours every morning.

Begin's closest loyalists -- most of them members of the "Fighting Family" drawn from the days of Begin's leadership of the Irgun Jewish underground in pre-independence Palestine -- portrayed in interviews a man in robust health, biding his time in semiseclusion before emerging to write his memoirs and bask in the glory of history.

Close friends say that the severe depression he suffered upon the death of his wife and during his last weeks in office appears to have lifted for the most part.

"His spirit is up 100 percent, and he has gained much of the weight he lost during those times. He is no less private, but he feels and looks better now than any time since he left office," a regular visitor said.

Other visitors say Begin is depressed over his wife's death and obsessed with the effects of the three-year Israeli war in Lebanon, which claimed the lives of more than 600 Israeli soldiers without attaining most of its goals.

The few friends and associates who see Begin regularly spoke on the condition that they not be identified.

Several frequent visitors to Begin's attractive stone apartment building in Jerusalem's fashionable Yefe Nof neighborhood -- situated on the side of a hill at the entrance to the Jerusalem Forest -- insisted that the truth lies somewhere between. They say that his moods change radically but that, on balance, he is a man at peace with himself, seeking solitude out of personal choice but prepared to emerge from his seclusion at a time of his own choosing.

Recalling that Begin three years ago could not bear to face the antiwar opponents who kept a vigil in front of his official residence with placards showing the Israeli death toll in Lebanon rising daily, one former adviser said the former prime minister recently has begun to study literature on the war.

Displaying a stack of magazines that Begin had just returned after reading and underlining articles on Israeli tactical and strategic errors in Lebanon, one close associate said, "You can speculate as far as you want about the effects that the war had on him, and he doesn't deny or confirm anything. But the fact that he is reading these articles suggests that he has a healthy, inquiring mind about the subject."

Begin, through his closest personal advisers, turned down a request for an interview.

Begin is said to begin his day at 5:30 a.m., first reading from a Bible that he keeps on a bedside table and then listening to Hebrew news on Israeli radio and English news on the BBC before reading two Hebrew morning newspapers and the English-language Jerusalem Post.

Then he dons a robe and sits throughout the morning before an open widow reading several more Israeli newspapers, The Times of London, The International Herald Tribune and several U.S. and European news magazines.

His daughter, Leah, who lives with him and cares for him, leaves the apartment before 7 a.m. for her job as a ground-staff employe with El Al Airlines; at 9:30 a.m., a housemaid arrives. About the same time, Kadishai usually arrives to take dictation of seven to 10 letters and convey requests for appointments.

Begin is visited by his son, Benny, nearly every day; by another daughter, Hassia, at least once a week -- sometimes accompanied by grandchildren -- and weekly by his former Cabinet secretary, Dan Meridor, currently a member of the Knesset, the Israeli parliament.

Visitors include Knesset member Haim Corfu, also a former Irgun member, and his wife, Devora, who live in the same apartment building; his former media adviser, Harry Hurwitz, his wife, Frieda, and longtime friends Nathan Silver, a Haifa businessman, and his wife, Lily.

When he receives visitors, Begin usually changes into trousers, an open-necked shirt and his robe, only occasionally donning the dark suit and tie that for years set him apart from his more casually dressed Cabinet ministers.

"He is not a hermit. He is seeing people. But he is living a very secluded life nonetheless, on his own terms. He almost never leaves the apartment, but that is his own choice. He never gives a reason why he doesn't go out," said one close friend of more than 30 years.

Associates say that Begin frequently answers his own phone, talking with friends and acquaintances both here and abroad, most recently with released Soviet prisoner Anatoly Shcharansky, and occasionally with Foreign Minister Yitzhak Shamir, who regularly visits the Begin apartment after making overseas trips.

Close aides say that Begin also frequently watches films on a video recorder, most of them rented from a neighborhood video club.

Although he has been offered advances of up to $750,000 for his memoirs, Begin has shown no interest in writing a book, his associates say. He was quoted as saying, "When I decide to write a book, you'll know it."