These are heady times for the UN, and its chief executive

When Javier P'erez de Cu'ellar suggested in January 1987 that he had no intention of presiding over the dissolution of the United Nations, skeptics shrugged it off as paraphrased Churchillian bravado. The UN, they predicted, would go the way of the British empire. For a long time, it looked as though they might be right. The Peruvian diplomat had just agreed reluctantly to accept a second five-year term as Secretary-General of an organization on the verge of political and financial ruin. The UN's public image had plunged to a new low amid frustration over its inability to make a dent in nagging global conflicts. And, as its chief executive, Mr. P'erez de Cu'ellar's stock had plummeted with the organization's.

Then overnight (as diplomatic gains are measured) both were back in the blue chips.

The upswing began last April with the signing of the Afghanistan peace accords, mediated by P'erez de Cu'ellar's special representative and successor at the task, Undersecretary-General Diego Cordovez. Next came Tehran's surprise acceptance on July 18 of the UN Security Council's year-old Iran-Iraq truce resolution, pointing the way toward the end of another devastating war. Scarcely three weeks later, P'erez de Cu'ellar came up with a peace proposal that promises to be the breakthrough in the 13-year-old Western Sahara desert war between Morocco and Algerian-backed rebels.

Now he is off on a new round of peacemaking. On Aug. 24-25, he will be in Geneva refereeing talks to which he has coaxed the reluctant leaders of Cyprus's divided ethnic Greek and Turkish communities. And on Aug. 25, he will initiate talks between the foreign ministers of Iran and Iraq.

There is also less tangible evidence that the UN - and its senior executive - are getting belated recognition. Last week, the Reagan administration, no great admirer of the UN or its chief, belatedly sent US Assistant Secretary of State Chester Crocker here to review the UN's comprehensive blueprint for steering Namibia (South-West Africa) toward independence.

Shortly before, South African President Pieter Botha invited P'erez de Cu'ellar to Pretoria to discuss the same subject. This, from a government that over the years has reviled the UN for having a leftist bias.

Despite recent success, the Secretary-General faces a daunting agenda. Today, the General Assembly resumes to resolve a fiscal crisis that could bankrupt the UN. And the Arab-Israeli conflict - one of the most enduring frustrations - shows no sign of solution.

Still, these are heady times for the man who, in 1982, became the fifth UN Secretary-General.

The Peruvian's diplomatic career has been wide-ranging. Among other things, he reminds his 15,000-strong staff that he is the first Secretary-General to have been ``one of you.'' He joined the UN Secretariat in 1975 after a tour of duty in Peru's diplomatic service that began in 1940. For more than 30 years his posts had taken him to Britain, France, Bolivia, Brazil and, as ambassador, to Switzerland, Venezuela, Poland, and the USSR.

He was a member of Lima's delegation to the first General Assembly session in 1946 and intermittently afterward. In 1971, he was appointed Peru's permanent representative to the UN.

Early assignments as a UN civil servant provided training for some of his current priorities. For three years he was the special representative on the Cyprus crisis. And, after being promoted to undersecretary-general in 1979, he twice initiated the UN mediation process on Afghanistan that climaxed in April.

On occasion, P'erez de Cu'ellar has angered both superpowers. Moscow was nettled by his dispatch of a team to investigate charges against its client state Vietnam of using ``yellow rain'' bacteriological warfare. Washington was annoyed when he criticized its expulsion of 25 Soviet diplomats.

P'erez de Cu'ellar has three hallmark traits: modesty, affability, and caution. He has raised ``sidewalk diplomacy'' to an art by stopping to talk with UN reporters in times of crisis.

But in recent days - perhaps, his apologists say, under the pressure of events - he has become testy and unnecessarily evasive with the press. And diplomats have complained that his caution frequently borders on timidity. For example, they criticize him for failing to react sharply when Ethiopia kicked the UN and private relief agencies out of its starvation-ridden country last spring.

He is sparing in expressions of optimism. Asked once if it was not an encouraging sign that two belligerents had agreed to talk, he replied: ``If two men meet together to disagree, I don't know what is the advantage. ...''

P'erez de Cu'ellar, considered the most intellectual UN chief since Sweden's Dag Hammarskjold, speaks fluent Spanish, English, and French. He and his Peruvian wife, Marcela, live in a mansion made available to Secretaries-General on Manhattan's fashionable east side.

In recent days, he has repeatedly brushed aside suggestions that he deserves to be a candidate for the Nobel Peace Prize.

Nevertheless, when Indian ambassador C.R. Gharekhan announced last week that P'erez de Cu'ellar had been named winner of the Jawaharlal Nehru Award for International Understanding, he added: ``And if I were chairman of the Nobel committee, I would have no hesitation in awarding him the peace prize.''