WASHINGTON (AP) _ In the blink of an eye, in a ceremony as old as the Republic, Ronald Reagan is transformed from Mr. President, the most powerful man in the world, to Mr. Citizen.

He keeps the title as an honorific, all the presidents do. But the mantle of office falls away completely. He becomes nothing ex-officio, he casts no votes, he has no say, and his counsel - if it is sought at all - is sought as a courtesy.

He becomes a member of a most exclusive club, that of ex-presidents, which will number more than three on Inauguration Day for the first time since 1861. The others are Reagan's immediate predecessors, Richard Nixon, Jimmy Carter and Gerald Ford.

Ronald Reagan has joked about what he will do immediately.

''As soon as I get home to California,'' he said, ''I plan to lean back, kick up my feet and take a long nap.''

That won't be so different from other recent presidents.

Harry Truman took the train home to Independence and said the first thing he did was to carry the luggage into the attic. The next morning he was up at 7:30 - having slept unusually late - and walked into his front yard to get the paper.

''My return to ordinary citizenship began a new life filled with new meaning,'' he said afterward. ''After all the crowds and furor, the whole business did not seem so easy. Were the people going to let me become a private citizen? Or was I still going to have to live up to the title of Mr. President that the people insisted on using?''

Truman was blessed with living in less dangerous times. At first, his only added security was the fence that the Secret Service had put up around the house at 219 North Delaware, and a former highway patrolman who doubled as chauffeur and bodyguard. He took his morning walks down to the town square often unnoticed. If you wanted to talk to him, you had to do it at 120 paces a minute, but you could do it. Only in his later years were laws passed that beefed up his protection at government expense.

Dwight D. Eisenhower, who hated top hats, had dictated the wearing of homburgs at his own inauguration. John F. Kennedy, aware of that, mischievously ruled that top hats be worn at his swearing-in and Eisenhower, as the outgoing president, was forced to comply.

No wonder he told reporters, as he was driven out of Washington, that it was wonderful to shed the burdens of the presidency. He and Mamie were driven in a four-car convoy to their farm in Gettysburg, Pa. The next day he attended a dinner thrown in his honor by the townsfolk and a couple of days later he was in Albany, Ga., shooting quail.

The latter was in the tradition set by Theodore Roosevelt who went on an African safari after leaving the jungles of Washington.

Ford didn't bother to go home. He stayed long enough to hear Jimmy Carter's inaugural address, got his helicopter pilot to make one last swing over the White House and the Capitol, flew to California and before the day was out was hitting golf balls off a practice tee.

The next day he was Arnold Palmer's partner in the Bing Crosby Pro-Am Tournament at Pebble Beach. Ford told reporters he thought he had prepared himself well for the swift switch in power. ''We certainly took the attitude that our time had ended and President Carter's time had begun,'' Ford said.

But he wiped away a tear or two at the airport, saying ''I don't deny I got a little sentimental, but I tried not to expose it, to keep it within myself.''

Carter's leave-taking and Nixon's - for different reasons - were the most traumatic.

Carter had stayed up all night handling final details of a complicated deal worked out with Iran to gain freedom for 52 American hostages who had been held hostage for 444 days. He catnapped on the Oval Office couch and left for the Reagan swearing-in only at the last minute.

En route to the Capitol, Carter got word the Iranians had agreed - but the first planeload did not take off from Tehran until five minutes into the Reagan presidency.

The 39th president flew to Georgia in Air Force One and to Plains in a helicopter. At 4:30 a.m. the next day he boarded the same aircraft for Germany to greet the returning hostages. Carter flew back home after a few hours on the ground.

Nixon, too, had a fitful last night in the White House. On Aug. 8, 1974, he had told the nation that because the Watergate scandal had eroded his political muscle ''I shall resign the presidency effective at noon tomorrow.''

Afterward, his family consoled him and Nixon went to bed with the sound of protesters outside chanting what his daughter described as ''their sick message.'' In the morning, he had corned beef hash and poached eggs alone in the Lincoln sitting room, signed a message of resignation addressed to the secretary of state, and said a tearful goodbye to his administration.

''This was the nightmare end of a long dream,'' he wrote later. ''I had come so far from the little house in Yorba Linda (Calif.) to this great house in Washington.''

He, too, flew back home on Air Force One, to years of self-imposed exile before he ventured forth again.

''There was no talk, there were no tears left,'' Nixon said of seeing the White House retreating in the distance. He leaned his head back against the seat and closed his eyes, then heard his wife saying, to no one in particular, ''It's so sad. It's so sad.''

Lyndon B. Johnson flew to Texas immediately after he left office on Jan. 20, 1969, landing in Austin before a crowd estimated at approximately 10,000 and then going on to the LBJ ranch in Johnson City.

In retirement, Johnson worked on his library at the University of Texas in Austin and writing his memoirs until his death on Jan. 22, 1973.