More than half the 66 Americans who were taken hostage at the U.S. Embassy in Tehran 15 months ago were mistreated either physically or psychologically after their capture, according to interviews with hostages, their families and government officials familiar with their ordeal.

Yet while there is evidence that the young militants who captured the Americans systematically tried to degrade, disorient and terrify their captives, most instances of abuse apparently were random incidents, springing from the whims of an individual guard or brought on by an act of defiancee or insolence on the part of a hostage.

These findings, coupled with interviews with experts and senior government officials who have reviewed the hostages' treatment, show that intimidation through physical brutality, as torture is frequently understood to mean, was apparently never employed.

But they also show that some instances of mistreatment may have been severe enough to qualify as "torture" as it is defined by both the United Nations and Amnesty International, a human rights organization that repeatedly criticized the late Shah Mohammad Reza Pahlavi's regime for tourturing its prisoners.

Nonetheless, there is some feeling in the State Department that former president Carter's emotional expression of outrage over the hostages' treatment after his meeting with the 52 just-released hostages in West Germany, coupled with initial news reports, created an exaggerated picture of the mistreatment the Americans received.

"[President Carter] was taken with the emotion of seeing all of them in the same room with him," said one senior State Department official. The official said he was "taken aback" by the first news accounts of the hostages' treatment because "they seemed to be much stronger than what I knew to be the case."

But, he said, there was no doubt that the hostages were from time to time treated so badly that it bordered on torture.

"How long do you have to have people's hands tied behind them for it to be torture?" he asked. " . . . Those people were held captive illegally and there was an element of torture in their captivity."

At the same time, however, he noted that "we did not have any type of sadistic beatings, systematic beatings."

As defined by the United Nations, torture is "any act by which severe pain or suffering, whether physical or mental, is intentionally inflicted" so as to intimidate, punish or obtain information or a confession. Amnesty International defines it as "the systematic and deliberate infliction of acute pain in any form."

The spectrum of mistreatment that has emerged from the hostages' reports in the last two weeks ranges from physical abuse, which most frequently occurred after escape attempts, to terror-inducing mock executions, to depriving the hostages of human contact, communication and the opportunity to bathe.

The worst period of physical and psychological abuse occurred during the two weeks immediately after the embassy takeover Nov. 14, 1979. Many hostages were bound to chairs, blindfolded for days at a time and threatened with death. Shoes were taken away, many were separated into groups or isolated from one another, and no talking was allowed.

The most serious of the abuses during this period occurred when the Iranian captors held guns to the heads of two embassy secretaries Terri Tedford and Elizabeth Montagne, and Alan B. Golacinski, a 30-year-old security officer, in an effort to force cooperation.

Several hostages, such as Robert O. Blucker, 52, an economics specialist, were also beaten during this time.

Today, however, it is possible to see that even during those first two weeks of general terror, the Iranians were marking some hostages for tougher treatment.

For instance, those hostages who held high posts in the political or economic sections, those who spoke Farsi, those who were high-ranking military officers and those suspected of spying were more likely to have suffered harsh treatment than consular officials or embassy guards.

Also, those who defied and insulted their captors or repeatedly tried to escape were subjected to particular abuse.

Two of the hostages kept hidden from the outside world during most of their 444 days of captivity and subjected to the most mistreatment were Malcolmm Kalp, the 43-year-old communications employe accused by the militants of being a spy, and Michael Metrinko, 34, a political officer fluent in Farsi and unremitting in his open hostility to his captors.

In a number of interviews, Metrinko described his continuous efforts to insult his captors and to thwart their wishes. "I just immediately started being impolite and refusing to obey orders and telling people to go to hell in general."

Metrinko, who also was accused of spying, was handcuffed for two weeks and kept in solitary confinement, he said. Metrinko told CBS News that he was beaten severely twice. "The two times [the beatings] were fairly serious, that was my fault," he said.

Kalp may have given his captors more trouble than any of the other hostages, however. He tried to escape on three occasions, managing to reach the outside of his building on one try before a watchdog gave an alert and the guards recaptured him. After the last of these attempts, he was put in solitary confinement for six months and beaten several times, according to the accounts he gave his relatives.

Another hostage beaten after an escape attempt was William Belk, the 44-year-old communications officer. "In my flight to escape I was shot at six or seven times," he said in an interview last week. In the course of his flight, he wrestled with a female guard, taking the ammunication clip from her automatic weapon.

After one escape attempt, Belk injured his leg. He was later kicked by militants in the injured leg, according to another account.

Among the other hostages beaten at one time or another, including Blucker, were John Graves, 53, an economics officer who was struck on the head during one of the mock executions; Charles Jones Jr., 40, who was destroying records when he was caught by militants and was kicked and beaten during subsequent interrogations, and Marine Sgt. John McKeel, 27, who told his father his captors knocked out one of his teeth during an interrogation.

Those kept in solitary confinement for a period during their captivity included both women hostages, Elizabeth Ann Swift, 40, the chief political officer, and Kathryn Koob, 42, the director of the Iran-American Society. Also held in solitary were a security officer, an Army sergeant who frequently fasted as a form of protest, a political officer who was told by the Iranians "we think you might be important," an administrative officer, the assistant Air Force attache and a communications officer who speaks fluent Farsi.

An abuse far more commonplace than beatings or solitary confinement however, was the day-to-day humiliation of dependency. Most hostages had to ask their captors for permission to go to the bathroom and were blindfolded when they were taken there; mail deliveries were made at the whim of the guards, and some hostages were taunted and shown letters they were never allowed to read.

In addition to many having their shoes removed and being forbidden to talk with their fellow captives, the hostages had to wear the same clothes for months at a time. Even when baths were allowed, some had to use the cold water left over from baths taken by other hostages.

Continual uncertainty was perhaps the most irritating of these persistent abuses: when Richard Morefield, the 50-year-old consul general, tried to get a clear definition of how long his letters could be or how frequently he could write, his captors were intentionally vague, Morefield said in an interview last week.

Cmdr. Don A. Sharer, a Navy flier, described his captivity as a Mental three-ring circum." At times, Shaver told The Norfolk, Va., Virginian-Pilot, the Iranians would throw birthday parties for hostages, complete with ice cream and cake. Then, a short time later, they would deny the same hostages mail.

While the mood changes of their captors was distressing, Sharer said, far worse were the unexplained, frequent moves, the nights of traveling blindfolded and trussed-up from one place to another.

"They would come in the room and say, 'Pack your stuff.' 'Why do we have to pack our stuff?' 'Just pack your stuff.' 'Are we going home?' 'Just pack your stuff.' 'Are you going to move us?' 'Just pack your stuff,'" he said. "Then all through your mind it would go, "They're going to take us out and shoot us.'"

Whether the hostages were ever tourtured, by the U.N. definition, the Amnesty International definition or any other definition, is clearly a moot subject now to many officials, however.

". . . They [the hostages] came out alive and physically well, except for a few cases of bronchitis," said the senior State Department official. "They suffered enormously, but they came out of it . . . They were strong enough to make it."