Retired Navy communications specialist John Anthony Walker Jr. and his son, Seaman Michael Lance Walker, pleaded guilty today to spying for the Soviet Union under a plea agreement in which John Walker will be sentenced to life in prison and his son will face 25 years in prison.

John Walker, 48, pleaded guilty to three counts of espionage in order to spare his son a lengthy prison term, according to to his lawyer, Fred Warren Bennett. Michael Schatzow, the chief prosecutor, said the government had agreed to a reduced sentence for Michael Walker, 22, in exchange for John Walker's promise to divulge what secrets he passed to the Soviets during 18 years of espionage.

The opportunity to debrief John Walker, Schatzow said, is "essential" for the government to assess precisely the damage caused by an espionage ring that authoritites have described as the most devastating in three decades.

As part of the plea agreement, John Walker agreed to testify if called at the Jan. 13 trial of Jerry Alfred Whitworth, the last defendant to face trial in the Walker case.

U.S. District Judge Alexander Harvey II said he would break from normal practice in Baltimore federal court and accept the arranged sentences because of the "exceptional circumstances in this particular case."

John Walker, a Norfolk private detective, will be eligible for parole after 10 years, and the government, as part of the plea agreement, promised to inform the parole commission of the "nature and extent" of his cooperation in the case.

But Assistant U.S. Attorney Schatzow, who prosecuted the cases against father and son, said it is extremely unlikely that John Walker will be paroled then. "It's theoretically possible, just as it's possible this building might fall on us -- and I think the chances of both happening are about the same," Schatzow said in a joint news conference with defense lawyers after the pleas were entered.

"I don't think he'll get out in 10 [years]. . . but he has a glimmer of hope" of being freed "before he dies," Bennett said.

Michael Walker, who pleaded guilty to five counts of espionage, will be eligible for parole after serving one-third of his sentence. Even if parole is denied, Walker would be free in 16 years and eight months -- before his 40th birthday -- with credit for good behavior, Bennett said.

In the plea arrangement, the government agreed to drop three of five counts of espionage against John Walker, the mastermind of the spy ring.

But it charged him in a new indictment, handed up Friday by a federal grand jury here and unsealed today, with a broad conspiracy to commit espionage from 1968 until May 21, the day after his arrest.

The new count, to which John Walker pleaded guilty today, accused him of conspiring with his son; his brother, Arthur James Walker, who is awaiting sentencing Nov. 12 on espionage charges; his Soviet contact, Aleksey Gavrilovich Tkachenko, and Whitworth, his close friend and Navy colleague.

Schatzow said the government sought the new count in order to illustrate the scope of the conspiracy. He said John Walker's admission that he conspired with Whitworth could be useful at Whitworth's trial.

Walker also pleaded guilty to attempting to deliver national defense information to a foreign government and to receiving national defense information unlawfully.

As part of the plea arrangement, Walker promised to "fully and truthfully disclose to the government everything he knows about espionage and espionage-related activities." The debriefing will probably start this week, Schatzow said, and it could take as long as a year.

Under the agreements with John and Michael Walker, the government promised not to court-martial them or accuse them of additional criminal charges, including income tax violations. The IRS has imposed more than $250,000 in civil tax liens against John Walker's property.

"It was a plea, but it was no bargain," Schatzow said after the three-hour hearing, which was attended by John Walker's daughter, Margaret; Michael Walker's wife, Rachel, and more than 100 reporters.

John Walker, wearing a gray three-piece suit, answered Judge Harvey's questions in a quiet voice. He smiled during much of the three-hour hearing, but his lawyer said later it was a "nervous" habit that did not reflect Walker's true feelings.

Bennett told Harvey that the "major reason" for his client's guilty plea was that he "loves his son very much" and wanted to "do everything" possible to seek lenient treatment for Michael Walker, who faced the possibility of life imprisonment. John Walker admitted that "he recruited his son and . . . was responsible, in many respects, for his son's conduct," Bennett said.

Bennett, who termed the government's case against John Walker "exceptionally strong, if not overwhelming," said Walker wanted to plead guilty as "an act of contrition" for his crime. "Mr. Walker does have feelings for his country," he said.

Michael Walker's lawyer, Charles G. Bernstein, told Harvey that his client "understands he has committed the gravest possible sin against his country" and "deeply regrets what has occurred." Michael Walker, dressed in brown pants and a beige shirt, responded softly to questions. At times during the testimony, he winked at his wife and smiled wryly.

Schatzow said the government agreed to the 25-year sentence for Michael Walker -- "considerably less" than what prosecutors would have sought had the case gone to trial -- in order to gain John Walker's cooperation in telling federal investigators what information he had passed to the Soviets.

"John Anthony Walker has something that . . . that government wants very much," he told Harvey. "We need to know what has been broken and what must be fixed" because of Walker's disclosures.

The government is already conducting a damage assessment based on a "worst-case scenario" regarding what occurred. But without John Walker's cooperation, Schatzow said, it would be impossible to obtain an accurate picture of what secrets had been compromised.

Michael Walker, the prosecutor said, "must be viewed as being less culpable than his father." Schatzow said the 25-year sentence "reflects the seriousness of what Michael Walker has done and the price . . . that the government is willing to pay to get the information that his father has." Schatzow said the plea agreements had been approved "at the highest levels" of the Justice Department and the Department of Defense.

Jury selection for John Walker's trial was to have taken place today, but it was abruptly canceled Friday. In the hearing that took place in its stead, Schatzow disclosed for the first time a detailed confession that Michael Walker gave to FBI agents shortly after his arrest.

In a 37-page statement of facts that the government would have presented had the men stood trial, which Schatzow read in court, the government said records seized in a search of John Walker's Norfolk house shortly after his May 20 arrest included a note indicating a payment to him of $24,500 for an exchange. The paper noted "that this is only half the usual payment . . . . "

An FBI official familiar with the case said later that agents had not been able to find large caches of money or other valuables belonging to Walker.

Also found in the search, the government said, was a Soviet-made palm-sized "rotor decryption device" that, combined with "key cards" Walker had access to during his 16-year Navy career, would have permitted the Soviets to "freely listen" to supposedly secure Navy communications.

Walker referred to other members of the ring with letter designations, with "S" standing for Michael Walker, for example. The note also discusses an "A," whose existence had not been previously made public. The FBI official, however, said agents were not "convinced A exists." He declined to elaborate.

The plea agreement revealed today was first raised as a possibility by the government in June, just weeks after the Walkers' arrests. According to Bennett, a draft agreement was drawn up June 25 after two "limited debriefing" sessions attended by John Walker, four FBI agents, Bennett and Schatzow.

Bennett said the government raised the possibility of a plea agreement and then became convinced that the plea agreement would be fruitful based on the information Walker provided during those meetings.

"We gave them a taste, a sample, but not enough" to reveal everything to the government, Bennett said. He said he heard nothing further for months about a possible plea agreement and continued to prepare for trial.

Last Wednesday, Bennett said, he received a telephone call from Schatzow reopening plea negotiations. A deal was drafted, and John Walker accepted it on Thursday. Michael Walker followed two days later.

Bennett said that if the case had gone to trial, he had planned to argue that the government had no proof that the Soviets had received documents from John Walker or that Walker intended to damage the security of the United States. Asked about the strength of such a case, Bennett shook his head. "Look, none of it was too pretty," he said.

John Walker and his son have communicated through letters over the past months, Bennett said. Michael Walker's lawyer, Bernstein, said his client has "some negative feelings [toward his father], which is understandable," but that Michael Walker is "gratified" by his father's agreement to plead guilty in order to secure a shorter prison sentence for his son.

"You look at this boy, and he's just not a spy," Bernstein said of Michael Walker, who rocked nervously in his court seat as Schatzow detailed the workings of the spy ring. "He regrets it."

Bennett described John Walker as a man who spent a lifetime waiting to be caught. "He's not depressed. He's not been a whiner . . . . He's sort of a fatalist. He sort of said he was looking to be tapped on the shoulder for this for 18 years."