WASHINGTON (AP) _ The one near-certainty in the Yurchenko affair is that ordinary citizens of the United States and Soviet Union will probably never know what really happened. And, under at least one possible scenario, even the two governments will never be certain.

For even now, a generation after an equally significant but less well publicized series of Soviet defectors arrived in the 1960s, retired CIA experts are still arguing over which ones told the truth and which were sent here to lie.

There are three basic possibilities:

- Vitaly Yurchenko, 50-year-old chief of KGB operations in the United States and Canada, was, as he claims, kidnapped in Rome, drugged, and brought to the United States. Once here, CIA officers initially drugged, threatened and tortured him to reveal secrets and later offered to make him a millionaire, but he escaped to the Soviet Embassy.

- Yurchenko voluntarily defected at the U.S embassy in Rome on Aug. 1 and has since served up a menu of Soviet intelligence secrets to CIA and FBI debriefers, as the State Department says. But for some undivulged reason - a love affair in the West gone sour, remorse, homesickness, fear for his family's fate - he changed his mind and returned home.

- As the No. 5 official in the Soviet KGB spy agency, Yurchenko and his colleagues plotted all along to stage a phony defection, to learn as much as he could about gaps in U.S. intelligence while giving up little, and to seize the first opportunity to go back and embarrass the CIA.

Based on interviews with present and past intelligence officials and experts and on the history of past cases, here is some of the evidence for and against each possibility:

- As for the first scenario, Yurchenko's version is flatly denied by the State Department and Senate Intelligence Committee chairman David Durenberger, R-Minn. They say the U.S. government does not resort to such strong-arm tactics. Other U.S. experts point out that his very ability to escape proves that he was not being held here against his will.

They add that he obviously made up his tale to avoid being executed for treason in the Soviet Union.

Yurchenko, though, never said the CIA kidnapped him; he said his CIA handlers claimed that ''friends'' brought him to them, with no explanation whether these were friends of the United States or friends of Yurchenko's.

Although they have never admitted doing it, U.S. officials have asserted the right under U.S. law, though not under foreign laws, to kidnap individuals abroad or have them kidnapped by others in order to bring them here to face criminal charges. Indeed, such claims were made when fugitive ex-CIA agent Edwin Wilson was tricked into flying back.

And CIA officials have testified in Congress that they kept another Soviet defector named Yuri Nosenko in a windowless, padded room for nearly three years beginning in 1964. They said they subjected him to hostile interrogation during that time while they manipulated the lighting to disorient his sense of time in an unsuccessful effort to get him to admit he had lied. Nosenko was later made a CIA consultant, a job Yurchenko said he was offered in the last three weeks.

In any case, if Yurchenko's story is true, only the U.S. government and Yurchenko himself know that for certain. The Soviet government could never be absolutely certain he didn't lie to cover up his temporary treason.

Experts say Yurchenko probably saved his life with the story: his execution would send the wrong message to other defectors who might like to return.

But, even if the Soviets believe him, they would probably have enough doubt never to give him as much responsibility as he had before.

- The second case, a voluntary defection followed by a change of mind, has the most U.S. adherents.

In support, U.S. intelligence sources said Tuesday that Yurchenko, who has a wife and son in Moscow, had a girlfriend in Canada. They said she was married to a Soviet diplomat there and suggested he defected to be near her but went back when their relationship turned sour. ''She liked him as a spy but not as a defector,'' one source said.

In further support of this view, they cite the relatively light surveillance that is usually accorded defectors, which would facilitate his escape.

Opponents of this view say he would not have implicated ex-CIA agent Edward L. Howard and another ex-U.S. intelligence official as Soviet agents unless he had been a genuine defector. Howard has fled the country rather than face espionage charges. The other man has not been identified publicly or charged.

But former counterintelligence experts disparage the quality of this information; one called it ''chicken feed,'' because the Soviets would have already gotten all they could from Howard and would lose nothing by compromising him in an effort to convince U.S. officials Yurchenko was a bona fide defector.

In any case, this is the one scenario in which neither government could ever be certain it knew the true story of the Yurchenko affair. Only Yurchenko himself could ever know all his motivations.

- The third case, a deliberately staged defection designed to lead to the embarrassment of the CIA, has some U.S. adherents, particularly among former counterintelligence officials.

The case against this is two-fold.

First, Yurchenko could not have been certain that he would have been given the freedom to escape that many defectors have had. He might have made a misstep and been locked up for three years like Nosenko.

Second, as chief of the KGB's entire intelligence apparatus in the United States and Canada, Yurchenko would have been too big a risk to send on such a mission. The Soviets would have had to worry that U.S. officials had perfected some drug which would make him spill far more secrets than he intended.

Although there is no public record of the use of any so-called ''truth serum'' on defectors in this country, the Soviets are aware of the disclosures that the CIA experimented for many years with mind-altering drugs.

Those who subscribe to the deliberate plant theory say that if Yurchenko had been genuine he would have revealed more valuable information than anything so far attributed to him.

They also point out that he appeared to be in charge during his news conference - three times brushing aside the suggestion by the Soviet minister- counselor Victor Isakov that the questioning end. One expert said this was not the picture of a man under a cloud or in disgrace but rather of a hero returning from a mission accomplished.

In the third case, the Soviets and Yurchenko would know the truth, but U.S. officials would never be certain.

This world of double and triple agents was summed up by author David C. Martin who entitled his book on the decade of turmoil fostered here by the 1960s defectors ''Wilderness of Mirrors.''