Vitaly Yurchenko, a senior official of the Soviet KGB hailed as the most important defector to the West in years, appeared at the Soviet residential compound in Washington last night to declare that he had not defected, but rather was kidnaped three months ago in Rome, drugged and held in forced isolation by the Central Intelligence Agency on an estate near Fredericksburg, Va.

Reagan administration officials immediately disputed Yurchenko's account, saying he came to the United States voluntarily and, after providing important information on KGB operations to his CIA debriefers, apparently developed second thoughts, walked out of his safe-house quarters and found his way to the Soviet Embassy Saturday night.

Sen. David F. Durenberger (R-Minn.), chairman of the Senate Select Committee on Intelligence, told reporters that Yurchenko was scheduled to come to dinner at the CIA Saturday night, but "never showed up."

A senior intelligence official said last night of Yurchenko's claim that he was kidnaped, "It's all a lie . . . . He defected and grew unhappy or it was a setup all along, but at this point we doubt it."

A senior intelligence source said last night that Yurchenko had passed a series of lie detector tests to establish his credibility as a defector.

State Department spokesman Charles Redman said last night, "At no time was Mr. Yurchenko held or coerced by improper, illegal or unethical means. It is Mr. Yurchenko's right to return to the Soviet Union once the United States government is, in fact, assured that this action is genuinely of his own choosing." A department official added that U.S. officials wanted to interview Yurchenko on "American soil" before he is allowed to leave the country.

Yurchenko, 49, said last night that he still has the Soviet diplomatic passport he was carrying when he traveled to Rome in July, and hopes to return as soon as possible to the Soviet Union to see his wife and son, 16.

An intelligence source said last night that Yurchenko had several personal reasons that might have led him to reconsider his defection, including a "girlfriend" in the Soviet Union.

Nervous and clearly under strain, Yurchenko appeared at an extraordinary news conference before scores of U.S. and Soviet reporters as well as dozens of Soviet Embassy personnel who showed pleasure at what they seemed to view as a propaganda victory two weeks before President Reagan and Soviet leader Mikhail Gorbachev are to meet in Geneva for their first summit. Yurchenko lost no opportunity to assail the United States, the CIA and the people who debriefed him here. Encouraged by a question from a Soviet journalist, he compared his abduction and treatment to an act of "state-sponsored terrorism."

Word of Yurchenko's news conference was first spread yesterday afternoon by Soviet Embassy officials, who told reporters at about 3 p.m. that the Reagan administration still did not know it was about to happen. Late in the afternoon, a Soviet diplomat delivered a formal protest to the State Department complaining of "the criminal act committed against V.S. Yurchenko," and asked that he be speedily returned to the Soviet Union.

Yesterday's sequence of events -- which seemed more suited to a spy novel than to real life -- culminated a week of highly emotional defection politics set off by a Soviet sailor in New Orleans, a Soviet soldier in Afghanistan and yesterday by Yurchenko.

The one-hour news conference in which Yurchenko spoke alternately in Russian and English left many key questions unanswered. He provided little detail about his "abduction," saying only: "I was forcibly abducted in Rome by some unknown persons. Unconscious, I was brought from Italy to the U.S.A. Here I was kept in isolation, forced to take some drugs and denied the possibility to get in touch with official Soviet representatives . . . ." He called his debriefing by the CIA "these three horrible months." He referred to his CIA debriefers -- two of whom he named -- as "torturers."

A State Department official countered, "He requested asylum in the U.S. and signed a statement to that effect, and asylum was granted. Since his arrival in the United States on Aug. 2, Mr. Yurchenko has willingly cooperated with both the Central Intelligence Agency and the FBI in providing information about Soviet intelligence activities throughout the world."

Yurchenko refused to discuss his reported status as a high-ranking KGB officer, but said he had been a "security officer" in the Soviet Embassy here during the 1970s. "I am not going to make any comments about spying business," he said. Last month the State Department identified Yurchenko as the former deputy chief of North American spy operations in the KGB's First Chief Directorate. He had previously been chief of worldwide counterintelligence.

Yurchenko would not provide details of his escape, but did describe his captivity at a CIA safe house on an estate near Fredericksburg.

On many other details he was hazy, describing himself as in a "fog" when he met one night for dinner at CIA headquarters in Langley with CIA Director William J. Casey. Yurchenko said he had the impression that his handlers were "trying to deceive the government, including Mr. Casey" into thinking he was a willing defector, and he claimed that they drugged him before that meeting. "I have only vague recollections of the conversation," he said, "but it was a kind of general conversation of big policy issues regarding the summit, things which they usually write about in the newspapers." Casey, Yurchenko added, "was evading effectual conversation about my case, and rather was dealing with generalities."

When asked if his defection and escape were an elaborate "sting" operation against the CIA, he said "no comment," but added, waving his hands in the air for emphasis, "I know your side now, better than reading 1,000 books."

From his account, Yurchenko said that his debriefing had "relaxed" in recent weeks and that CIA officials had tried to get his signature on a contract that would pay him a $1 million down payment and $62,500 a year -- with an inflation adjustment -- to live in secrecy and perform consulting services for U.S. intelligence. The consulting deal also included $48,000 worth of furniture from the safe house, he said.

Such an arrangement would be consistent with what is known about how the CIA treats Soviet defectors. Arkady Shevchenko, for example -- the deputy secretary general of the United Nations who defected in the mid-1970s -- received a lifetime annual stipend of $60,000 a year.

Yurchenko claimed he was threatened by his debriefers and tempted with the lucrative contract to live as a secret consultant to the CIA. "They told me, 'If you flee, prison and death are waiting for you. We'll send the KGB all the material we got from you,' " he said.

Yurchenko may have been using yesterday's news conference to try to ease the usually harsh Soviet retaliations against families of defectors, intelligence sources said. Yet he did not play the role of a conventional Soviet official. He appeared eager to respond to reporters' questions, and more than once ignored the guidance of Embassy Minister Viktor Isakov, who sat beside him. The other Soviet officials in the room appeared uneasy when Yurchenko was pointedly asked how he could have failed to disclose Soviet state secrets during three months of debriefing by U.S. intelligence officials.

In reply, Yurchenko said emphatically, "In the period when I was conscious and controlled my behavior, I did not pass any secret information." But he added, "I did not know what I was saying or doing when I was drugged." Yurchenko said his handlers "showed me documents which were written in my hand" and said "they had recordings of my conversations," but "nevertheless, I did not believe them."

In the course of the news conference, Yurchenko tried to disavow the most important intelligence discoveries that have been attributed to him by U.S. intelligence officials.

The first involved a former officer of the CIA's clandestine service, Edward Lee Howard, who the FBI has said was a spy for the Soviet Union. Howard joined the CIA in 1981, was trained to become a clandestine operative in Moscow, and was fired in June 1983 for failing to stop his occasional drug use.

In late September, Howard was put under surveillance by the FBI because of information provided by Yurchenko, according to government officials. But Howard escaped before a warrant was obtained for his arrest on espionage charges.

Information Howard provided to the Soviets allegedly helped them uncover a CIA "asset" in Moscow -- a Soviet aviation expert named A.G. Tolkachev who had been providing valuable information on Soviet electronic weapons research.

Tolkachev disappeared in early June, and the Soviets subsequently expelled a U.S. Embassy political officer, Paul Stombaugh, whom they accused of carrying on espionage activities in Moscow. Stombaugh reportedly was Tolkachev's case officer in the Soviet capital.

Asked about this yesterday, Yurchenko said, "The first time I recognized the name Howard" was when his CIA handlers brought him newspaper accounts of his alleged spy role and disappearance. However, U.S. officials had previously said that Yurchenko had known Howard only by a code name, but provided details about him that helped the CIA and FBI to expose him.

Yurchenko also claimed not to be the source of the CIA's confirmation of the death of Nicholas G. Shadrin, a CIA double agent who disappeared in Vienna in December 1975 on an evening he was to meet KGB officials. Just a week ago, acting on Yurchenko's information, the FBI sent two agents to the home of Shadrin's widow to inform her that her husband was dead. And last week -- also using Yurchenko's information, according to intelligence sources -- the State Department called in the second-ranking official of the Soviet Embassy here to read him a formal protest. It accused the Soviets of a "reprehensible" crime in kidnaping and killing Shadrin, and further accused the Soviets of lying about the matter repeatedly since Shadrin's disappearance.

"I don't know [if Shadrin is dead]," Yurchenko shouted to reporters at the end of the news conference.

Once, late in the news conference, Yurchenko paused, cleared his throat, and when he looked up at reporters, his eyes watered as he apologized for his rambling statements and his fatigue. He attributed them to the "helpless" months he spent as a captive and many sleepless nights in which, he alleged, he could not close the door to his bedroom, which was guarded by what he described as a "fat, quiet and stupid" security officer.

Several members of the congressional intelligence oversight committees said last night that Yurchenko got "cold feet" after providing important information about the KGB's worldwide operations.

"His debriefing information is for the most part reliable," said Sen. Durenberger. "The stuff that we have been able to check, has checked out."

Durenberger said he could not explain the bizarre circumstances that led to Yurchenko's return to the Soviet camp. "He is a pro. He knows whereof he speaks and the information he has given us is all valid information. So there's something else that doesn't click here. Either this was a setup from the beginning just to make us look bad . . . , or the guy really is under some substantial duress and thinks that somehow he is feathering a nest in Siberia or for himself . . . ."

Sen. Patrick J. Leahy, (D-Vt.) vice chairman of the Senate panel, said last night that he believed Yurchenko was either trying to get back into the good graces of Soviet officials to protect his family, "or he was a double agent all along."

Sen. William S. Cohen (R-Maine), a member of the Senate intelligence panel, said last night that he and two other senators expressed doubts to high CIA officials about whether Yurchenko was an authentic defector.

"There was skepticism from the beginning," Cohen said, "because his defection seemed too pat, too convenient. It raised a number of red flags on the committee . . . . We kept asking, 'Are you sure this guy is real?' "

Yet at the same time, CIA officials treated the Yurchenko defection as the biggest intelligence coup of the decade, one that would set back Soviet intelligence for years to come.

Cohen said the intelligence committees got no notice that Yurchenko had returned to Soviet custody and doubted any charges that drugs were used in his debriefing.

"He had to say that to save his fanny," Cohen said of the drug allegations. "It's conceivable the guy is unstable and had a change of heart. It's plausible that he was acting as a double from the beginning."

Some of Yurchenko's statements did appear inconsistent. He alleged that his CIA interrogators kept him incommunicado and in isolation, but he greeted one reporter's question by exclaiming that he had received several letters from the reporter. Yurchenko said the reporter's letters were forwarded to him by CIA officials.

Pressed to explain how he had gotten away from the CIA safe house, Yurchenko offered this reply: " . . . Well, I find myself in really tough situation. I have a 16-year-old son and he has his problems with his studies and with his behavior . . . . I used to tell him that there can be no situation with no way out. If you really think about it you can always find a way out of any situation . . . ."

Staff writer Caryle Murphy last night located a house north of Fredericksburg that may have been the safe house where Yurchenko was debriefed over the last three months. Located in a subdivision called Coventry, a name Yurchenko mentioned twice in his news conference, it is a large new home on a large lot on a lake -- also mentioned by Yurchenko.

[Only nine homes have been built in the wooded subdivision. Two residents said last night that one of the houses had been rented this summer by its owners to a lawyer who said he did not want to live there but wanted to use it for "his corporation." Reached late last night, the owner of that house declined to discuss the matter. The house seemed empty, and no one answered the doorbell.]