President Hafez Assad of Syria never performs better than when the pressure is on and his long career sometimes seems like an uninterrupted series of challenges and crises.

The most notable traits of the 55-year-old former Air Force officer, sometimes referred to as the "Bismarck of the Middle East," are public silence, tireless energy, personal austerity, a wry sense of humor, attention to detail and an ability to wear down interlocutors in the late night working sessions he favors.

Throughout the hostage crisis, the only departure from form was the uncharacteristically imprudent communique Saturday announcing the Americans' release had taken place a day before he was finally able to bring it about.

Yet such was his confidence that he could begin a state visit to Czechoslovakia on Tuesday that he announced his trip to Prague last Wednesday, in effect setting himself a deadline and telling the world the crisis would be over by then.

In his memoirs, former secretary of state Henry A. Kissinger drew a flattering portrait of Assad, whom he clearly came to admire during the grueling shuttle diplomacy Kissinger conducted between Jerusalem and Damascus after the 1973 Arab-Israeli war.

Born in a village near the Mediterranean port of Latakia to a family opposed to French rule under the League of Nations mandate, Assad became an ardent nationalist while still a schoolboy.

As did so many other members of his minority Alawite Moslem sect, he decided on a military career, a vocation the majority mainstream Sunni Moslems traditionally disdained.

After graduating from the Air Force academy, Assad joined the smallest of Syria's military branches and about that same time he became a member of the Baath (Arab Renaissance) Party.

The Baath, because of its secular ideology, was attractive to many Christians and other minorities within Syria.

When the Baath, with its many Alawite supporters, seized power in 1963, Assad already was well placed in the Air Force. Four years later, during the six-day war with Israel, he was minister of defense.

The Syrian Baath Party became embroiled in a power struggle between a radical civilian group and a more moderate military faction. In September 1970, when fighting broke out in neighboring Jordan between Palestinian guerrillas and King Hussein's Army, Assad took advantage of that conflict to make a masterful move for political control in Syria.

He refused to commit the Air Force to protect Syrian armored units that invaded Jordan to help the Palestinians, thereby dooming that foray and the civilian party leaders who had ordered it.

His power was never again significantly challenged from within Baath party ranks.

In dealing with his adversaries he has proved that he does not shrink from overwhelming use of force. What is thought to have been a very large number of Syrians were killed when his forces put down an uprising led by the fundamentalist Moslem Brotherhood in the Sunni stronghold of Hama in February 1982.

Yet the greatest threat to his rule was caused by the heart attack he suffered Nov. 13, 1983. In its wake -- and much to his ill-concealed displeasure -- there was an unseemly struggle for the role of his heir-apparent.

Nevertheless, by mid-1984, Assad's control of his government appeared, if anything, to have increased. And with every passing month the speculation about his health has diminished, although at times he appears frail.

And while Assad's once legendary 18-hour working days are said to be a thing of the past, during the hostage crisis lights at the presidential palace burned late into the night.