RONALD REAGAN'S first reaction to the news that the Soviets had shot down a Korean airliner was, as one adviser put it, to get "mad as hell" and want to do something about it. His second and almost simultaneous response was to decide to spend the day at his mountaintop retreat northwest of Santa Barbara rather than return to Washington to denounce the Soviets.

Reagan's advisers subsequently convinced him to cut short his vacation and fly back to the White House for symbolic reasons. But Reagan's reluctance to leave his 688-acre spread in the Santa Ynez mountains even in the midst of an international crisis provided a rare public glimpse of the private conflict the president appears to have experienced as he decides on a second term.

Most of the president's close advisers say he is running. One of them last week said that there was only "a 5 percent chance" that he would decide not to seek re-election.

But the fact that there is any room at all for doubt about the second-term intentions of a president who likes his job and is without a serious rival within his own party is a reminder that Reagan, unlike many other recent presidents, has a genuine talent for appreciating life outside of Washington. "The decision to run again is a decision not to forego this (ranch) life but to lead far less of it," says a Reagan intimate.

Reagan is still healthy at 72. But a sense that for him, a decision to run again amounts to a decision to spend what could be the rest of his productive life in Washington captivity, crept into a recent interview. He referred to a second term as "the last hurrah."

Ronald Reagan is already the oldest president in the nation's history. If re-elected, he would be 78 within three weeks of leaving office in 1989.

Yet Reagan's desire to stay in Santa Barbara reflected the personal inclinations of a man who likes physical activity and enjoys horseback riding with Nancy Reagan along the freshly pruned trails that wind through Rancho del Cielo, his aptly named "Heavenly Ranch."

He finds relaxation chopping and splitting the winter-damaged branches of old trees for a firewood supply. He finds it, too, in building a 4-foot-high fence out of of old telephone poles -- a fence that he plotted meticulously on drafting paper.

Questioned by Ann Devroy of Gannett News Service recently about why he enjoys hard work around the ranch, Reagan described the presidency as a "sit-down job."

"When you are working the hardest, you are sitting the longest," he said.

What skepticism exists about whether Reagan will finally decide to run again for president is based on the recognition that he really enjoys what Nancy Reagan is believed to consider a hard life on the ranch. The reasons that other people might find for Reagan to step down -- such as the likelihood that he would have to raise taxes, not lower them, in a second term, and the growing clouds of international crises -- do not seem to have intruded into the presidential consciousness.

"Reagan is such an optimist that he always thinks he will come out ahead," says a close adviser. "Besides, he thinks he'd handle the tough problems better than other people."

Maybe so. In any event Reagan is not in the habit of confiding his decision-making processes even to close intimates.

When he first ran for governor of California in 1966 and when he ran for president in 1976, he constructed escape hatches for himself in the form of "exploratory committees" that would have allowed him to withdraw if his campaigns had failed to make headway. To this day Reagan bristles when someone refers to his abortive race for the presidency in 1968 as a campaign. The fiction preferred by Reagan is that he did not really run at all and was just making himself available in case Richard Nixon faltered.

One Reagan supporter said this approach is a reflection of the president's competitive spirit and unwillingness to lose, adding "if he didn't run, he didn't lose." This may be a reason why Reagan is reluctant to become a candidate for a second term until the last minute. "The people will tell me whether I should run again," he is fond of saying when asked.

This time Reagan may have more to lose than a re-election fight. The 1984 "last hurrah" could be a verdict not only on his performance over the last four years but on his entire range of ideas -- from shrinking the size of government to reasserting American power in the world -- for which he has been crusading for two decades or more. It will be a verdict on whether Reagan could make conservatism work.

White House colleagues say there is a special intensity to Reagan's feelings on this point -- a sense of destiny that was reinforced by the assassination attempt in March, 1981. "There is a certain determinism in his own life," said one long-standing associate. "The assassination attempt reinforced it."

The sought-after "ratification" of Reagan's ideas is hardly complete after his first 2 1/2 years in the White House. Indeed, many Reagan concepts have been proven more wrong than right. Not even Reagan claims any more, for example, that tax cuts will produce more revenue than they will cost.

Certainly, some of Reagan's most dearly held goals are now far beyond his grasp, and others remain tantalizingly elusive. He wanted to balance the budget and instead is buried under an avalanche of deficits. He wanted to show that a defense buildup would bring about nuclear arms reductions, but an arms deal is not yet in sight.

Reagan will not get another chance in his lifetime to demonstrate that "peace through strength" can work. He will not get another chance to show that the "evil empire" of the Soviet Union can be brought to the nuclear arms negotiating table by the threat of building up American military strength. This goal of an arms treaty, particularly, is one Reagan discusses privately as a chance to leave behind a ratification of his views of how the world works.

His desire for vindication is more publicly acknowledged when it comes to economic policies. In the nascent phases of his re-election campaign, he has been advertising economic recovery and low inflation as proof that his tax and spending cuts were the right medicine for an ailing nation. "You don't hear them calling it Reaganomics anymore," he crows.

Yet Reagan rarely gives evidence of any soul-searching. Instead, Reagan is repeating a pattern set earlier in his political career when he first fashioned himself a "citizen politician." Even to this day, a White House assistant says, "He doesn't think of himself as a politician."

In trying to assess what Reagan will decide about running again for president, there is an unspoken assumption that he will indeed make a decision at some key point. This is not necessarily correct. Though Reagan can be most decisive in a crisis, he is by nature a passive man. This passivity was deepened by his years in Hollywood where Reagan -- no prima donna -- was known as an actor who took direction well and did what he was told.

The present campaign law is made to order for a second-term try by a passive president. It requires the candidate to make a decision not to run once his campaign committee has filed. There is no question that the committee will be formed. It is not "the people" Reagan talks about, but a coterie of close associates who have made preparations for a second- term campaign. They will not be dissuaded except by an outright veto by Reagan. In 1966, Reagan was talked into running for California governor by a group of three wealthy Republican businessmen who had been backers of Sen. Barry Goldwater's ill- fated presidential campaign.

In fact, the only decision for Reagan to make now, according to several political operatives who are readying the re-election effort, is whether he wants to bail out. And said this , unless his health fails or hispresidency crumbles unexpectedly, they don't expect him to retire voluntarily to the ranch. Reagan has said as much himself recently. Asked whether the ranch would become his retirement home, he said, "I can't see me sitting on the front porch in a rocking chair."

No one in the Reagan political circle believes that re-election will be a breeze. Some advisers think the contest could be very close. And the aides who are looking ahead to what a second term might bring do not find the prospects for success as promising as in his first two years.

One aide observes:

"The prospects of leaving office in 1988 with the same approval rating as he has today are slim to none. The prospect of the same kind of successes as Reagan had in his first two years are slim to none."

In a second term, this adviser says, "You may be faced with another recession. You have the possibility of a tax increase -- and a big one -- to deal with the deficit. You might have a new and more aggressive Speaker. You may lose the Senate. You will probably have a more recalcitrant Congress. You certainly will have less good will."

But Reagan is bored by political calculations and remembers, as an aide points out, that he was as successful with Democrat-controlled legislatures in California as with Republican ones.

None of these political calculations is likely to deter Reagan from running again. What would deter him, some intimates believe, is if he had to choose between the power of the presidency and the pleasures of his Santa Barbara ranch. However, he doesn't necessarily have to make that choice.

In less than three years of his first term, Reagan has made 18 trips to California, 16 of them to his beloved ranch. Most Reagan watchers think he would spend even more time there during a second term when he wouldn't have to concern himself with critics who contend he spends too much time vacationing.

"What," an adviser was asked hypothetically, "would Reagan do if the Soviets shot down an airliner during his second term?"

"The same thing he did this time," was the reply. "Only he'd stay in Santa Barbara the next time."