Former national security adviser John M. Poindexter testified yesterday that in his final conversations with the president, attorney general and White House chief of staff last November, none of them asked what he knew about the diversion of U.S.-Iran arms sales proceeds to the Nicaraguan rebels.

Poindexter testified further that he offered no explanation to them why he decided the previous February to approve the diversion plan without telling the president or other senior officials what he had done.

That testimony, along with other new assertions, was elicited by Iran-contra probers whose questions yesterday challenged Poindexter's credibility and the plausibility of his dramatic assertion Wednesday that he never told President Reagan of the diversion because he wanted to protect him from political damage if it ever became public.

That account was hailed by the White House as corroborating Reagan's own version of events.

But a skeptical Arthur L. Liman, chief counsel of the Senate Iran-contra panel, asked Poindexter why it had taken him eight months to reveal a piece of information that would have helped exonerate a president Poindexter said he wanted to protect. "Can you tell us why you didn't then {last November} stand up and say, 'I, Adm. Poindexter, made the decision and did not tell the president of the United States'?" Liman asked.

Poindexter responded: "That's a hypothetical question and before I made the decision to retain attorneys, I was obviously giving it a lot of thought."

Several members of the Iran-contra panels yesterday expressed skepticism about Poindexter's testimony. Rep. Louis Stokes (D-Ohio), chairman of the House Select Committee on Intelligence, said the Navy rear admiral's account of the diversion of funds "doesn't square with me."

Sen. William S. Cohen (R-Maine) questioned the credibility of Poindexter's assertion that the diversion of funds was a technicality. Sen. George J. Mitchell (D-Maine) said he was concerned about Poindexter's oft-stated inability to recall key facts.

Until he was granted limited immunity from prosecution, Poindexter, exercising his Fifth Amendment rights against self-incrimination, had refused to answer any questions. He is a target of a criminal investigation run by independent counsel Lawrence E. Walsh, and can also be prosecuted for perjury if he lies in his sworn testimony before the congressional panels.

In three hours of interrogation apparently intended to highlight Poindexter's prior actions to protect the president, the witness said he:Destroyed documents that Liman said were sought by Attorney General Edwin Meese III. Made no effort to prevent then-White House aide Oliver L. North from destroying a notebook relevant to the same Meese inquiry. Hinted that other documents should be done away with. And admitted at one point that his "objective all along was to withhold information from Congress" regarding the National Security Council's activities in support of the contras.

In the most striking conflict to date between his testimony and North's, Poindexter said he did not recall a conversation with North last Nov. 21 that was a key element of the Marine lieutenant colonel's account of events when the Iran-contra affair unraveled.

North testified that on Nov. 21, he asked Poindexter directly if Reagan knew about the diversion of arms-sales profits to the contras, and that Poindexter said Reagan did not know. Until that moment, North said, he had believed the president knew and approved what he was doing.

"I heard his testimony" about the Nov. 21 conversation, Poindexter said. "I don't recall that."

"Do you recall him asking at any time about the subject?" Liman pressed.

"I -- I simply don't recall it. He -- he may have, but I -- I don't recall it."

Poindexter appeared to slightly shift his testimony about five memos North testified he forwarded to Poindexter to get presidential approval for forthcoming arms shipments to Iran -- three of which ultimately took place, acknowledging indirectly that they probably did exist.

North said each of the five memos contained a reference to the amount of proceeds from a proposed arms sale to Iran that would go to helping the Nicaraguan rebels. Poindexter on Wednesday first said he could not recall receiving any such memos, but later said he might have received one. Yesterday he repeated that but said North's "normal practice" was to send him a memo before each arms transaction, and "I would use the paper that Col. North prepared as talking points."

"If it was very detailed and I thought the president might be interested in it, I would sometimes read it. But more often, I would summarize it."

North has testified that he thought he had destroyed all of these documents, but one was found in his files by Justice Department officials Nov. 22. It was the discovery of that memo that led to the firing of North and the resignation of Poindexter.

For the first time yesterday, Poindexter described his meetings with top administration officials and the president in the aftermath of the discovery of that North memo.

On Monday, Nov. 24, Poindexter said Meese came to see him in his White House office. According to other testimony, Meese that day had asked his criminal lawyers to look into the legal implications of the memo.

But during what Poindexter described as a "very brief conversation," the attorney general mentioned the memo found in North's files and asked if Poindexter was aware of it.

"I said I was generally aware of -- of the transfer of funds, or the plan to transfer funds, and then I told him, I told him I was prepared to resign, and that I trusted him to recommend to me the timing of my resignation and that was essentially the end of the conversation," said the witness.

Liman asked whether he told Meese of his approval of the diversion by North, and Poindexter replied: "I did not use those words. I told him I was generally aware of the -- of the plan to transfer the funds."

"I was being very cautious at that point," he said.

"Is it a fact that the attorney general didn't ask you whether you had told the president?" Liman asked.

"I do not recall his asking me whether I had told the president or his asking me whether I had approved it," said Poindexter.

The next morning he briefly saw Meese, who said "the time had come" for him to resign.

"I said, 'Fine,' " Poindexter said.

When Donald T. Regan, then the White House chief of staff, stopped by his office shortly afterwards, Poindexter said, he could not recall whether the president's top aide asked what he knew about the diversion or whether he had told the president anything about it.

Minutes later, he said, he walked into the Oval Office and, in the presence of the president, Vice President Bush, Regan and Meese, announced: "Mr. President, I assume you are aware of the paper that Ed Meese has found that reveals a plan to transfer funds to the contras. I was generally aware of the plan and I would like to submit my resignation to give you the latitude to do whatever you need to do."

"And the president responded and said that he had great regret, and that was in the tradition of a naval officer, of accepting responsibility. And I shook hands with everybody and left the office. That was the last time I saw the president."

Two hours later, the president and Meese told a televised news conference that Poindexter had resigned and North had been dismissed. Meese told the news conference that "Poindexter did know that something of this nature was occurring but he did not look into it further."

One mystery that deepened yesterday as a result of Poindexter's answers has to do with the money generated by the arms sales.

Although Poindexter and North have testified that they were aware of the enormous political risks involved -- drastic enough that they kept the scheme from the president, they said -- they have both claimed to be ignorant of how much money the plan generated, and how much actually went to the contras.

According to Poindexter, it was his "understanding" that all the profits were going to the contras. Under questioning yesterday, however, he acknowledged that he had never asked how much money the arms sales were generating and that North had never told him.

When asked why he had not inquired, he said, "I simply did not get into that detail of micromanagement of the project that Col. North was working on. I've told you, that was not my style."

Profits from the sale of U.S. weapons to Iran in 1986 were funneled into Swiss bank accounts controlled by North's hand-picked "commercial cut out," retired Air Force major general Richard V. Secord, and his business partner, Albert A. Hakim.

Earlier testimony has established that the arms sales generated some $18.3 million in gross profits. Support for the contras paid for by those funds totaled just $4 million -- almost all of it in the form of support to an airlift operation also controlled by Secord and others.

More than $8 million is still unspent, and now sits in frozen foreign bank accounts. North himself acknowledged that he was "shocked" to learn of this.

Poindexter's testimony yesterday included new details about his destruction of documents last Nov. 21 after he learned that Meese was launching a fact-finding inquiry into the 1985 shipments of arms to Iran. Wednesday Poindexter testified that he tore up the only signed copy of the presidential intelligence "finding" that provided post-facto authorization for 1985 arms shipments and authority for future shipments, too.

Yesterday Poindexter said he also destroyed copies of messages outlining points "that I wanted to brief the president on" regarding the secret Iran initiative. "I saw nothing in them that we needed at that point," he said, echoing the explanation he gave Wednesday for destroying the finding.

On another occasion, Poindexter said yesterday, Kenneth E. DeGraffenreid, then of the NSC staff, reminded him that there were sensitive memorandums describing North's activities on behalf of the contras in the files. This happened after the Nicaraguan government last Oct. 5 shot down an airplane involved in the secret contra resupply effort coordinated by North.

Poindexter said the memos "probably did reveal too much and that he should talk to Col. North about them." He said the implication of his remarks was "that we probably would be better off without those memos."

The White House yesterday disputed Poindexter's earlier contention that Reagan would have approved the diversion of funds to the contras if he had been asked to do so. Spokesman Marlin Fitzwater said the diversion "wouldn't have happened" had Reagan been advised.

Fitzwater also said that officials who failed to seek Reagan's approval for important decisions -- as Poindexter testified he had in the case of the diversion -- had done the president a "disservice."