After two days of testimony, Rear Adm. John M. Poindexter has left the Iran-contra congressional committees and the American people grappling with a central question: How plausible is the "plausible deniability" he says he wanted to give President Reagan?

Far from resolving that issue, Poindexter's testimony yesterday raised new questions about it.

He's asking the committees to believe not only his word but the credibility of his actions. Take, for example, his description of the events of Nov. 21, a critical day for the administration as it groped for a way to contain the damage being caused by the unraveling Iran arms sales scandal.

To set the scene: At that time, only the Iran arms sale was known to the public; the contra diversion was not disclosed by Attorney General Edwin Meese III until the next Tuesday, the day Poindexter resigned as the president's national security adviser and Lt. Col Oliver L. North was fired.

As Poindexter testified yesterday, that Friday morning began with a meeting in the Oval Office involving Meese, Reagan and Poindexter. They were discussing discrepancies that had arisen about the November 1985 shipment of Hawk missiles from Israel to Iran, and the U.S. role in that shipment, Poindexter said. Also at issue that morning was how they would deal with the question of when the president had approved such a shipment.

Two nights before, Reagan had held a disastrous televised news conference, in which he had said that "no third country" had had a role in the transfer of arms to Iran. The White House corrected that statement 28 minutes after the news conference, saying that a third country had indeed been involved.

Now, Poindexter testified yesterday, "The question was of the president's prior knowledge . . . and whether he had approved it or not, and we needed to get to the bottom of that."

John W. Nields Jr., chief counsel for the House panel, immediately asked the admiral: "Did anyone at that meeting ask the president whether he had approved?"

"That's an obvious question," Poindexter testified, "but I don't recall it coming up . . . and I don't recall the attorney general asking him about {it}."

From Poindexter's account, there did not seem to be many questions asked during that meeting. All

the participants presumably had knowledge of the decision to approve the shipment, and Poindexter was asking the committees to believe that none of them remember that involvement. On such a crucial decision, he was suggesting, all had been forgotten.

Poindexter himself had brought the "finding" to the president for his signature.

Reagan signed it Dec. 5, 1985, belatedly authorizing the shipment that had taken place the month before.

Meese had worked on two subsequent findings that superseded the Dec. 5 action, which the president signed on Jan. 6, 1986, and Jan. 17, and presumably had been told of the original finding. These new findings broadened the stated purposes of the arms deal from a straight arms-for-hostages swap to greater geopolitical strategic objectives in the Middle East.

Something else came out of that meeting -- the decision to assign Meese to conduct a "fact-finding" inquiry into how the arms deal had occurred. As it was described yesterday by Poindexter, the idea was casual. "Ed asked the president permission to do it, and the president said, 'Fine,' " Poindexter said.

He also testified that he viewed Meese as acting "in his role as really a special adviser to the president." This sounded like North's testimony last week that Meese was acting as a "friend" of the president.

The meeting concluded, the congressional investigators were told yesterday, with Poindexter returning to his office. Meese then telephoned him and asked that he gather appropriate documents relating to the arms sale and have them ready for two aides to review later that day.

This news prompted Poindexter to retrieve the original Dec. 5 finding, and the two subsequent ones along with some PROF computer notes from an NSC safe -- and destroy them all before the attorney general's aides arrived.

Poindexter described this action graphically, saying he "ripped up" the finding and put it in the "burn basket behind my desk."

Nields wanted to know what motivated him to destroy the document that answered the question of the origins and the authorization for the arms sale -- the main purpose of the morning meeting at the White House that led to the Meese inquiry. "Did you destroy it to give the president deniability?" he asked.

"I did not."

"Well, did you . . . not, by destroying that document, give the president the ability to deny that he had signed a finding, on Dec. 5th, that dealt with arms and hostages?"

This led to a stormy objection from Poindexter's lawyer, Richard W. Beckler, who said: "I submit it's been asked over and over."

The previous day, Poindexter had said his intention had been to save the president from "significant political embarrassment."

Beckler's objection was overruled by House Chairman Lee H. Hamilton (D-Ind.), and Nields resumed the same line of questioning: "By destroying this document, did you not permit the president to deny that he had signed a finding on Dec. 5, 1985, relating to arms and hostages?"

"That was not the intent at all," Poindexter maintained.

He added, in response to further questioning: "I . . . simply did not want this document to see the light of day."

"{Were you} attempting to falsify the record?" Nields asked.

"No," Poindexter insisted, "I wasn't at all."

After another heated objection from Poindexter's lawyer to shift the questioning to other subjects, Nields asked:

"Adm. Poindexter, what made you believe that the president of the United States would want you to destroy a finding in order to save him from political embarrassment, if anything?"

"I -- I -- that -- that thought didn't cross my mind. I recognized that it was politically embarrassing. I thought one of my jobs was to protect the president, and I didn't think about asking him about it."

The questioning on this crucial subject ended with Nields shifting the nature of the "deniability" factor from the president to others -- including Poindexter himself.

"I have to ask you this question," he asked Poindexter. "Was it any part of your purpose in destroying that document to protect anyone other than the president?"

"Nobody," the admiral replied.

"And, particularly, was it part of your purpose to protect yourself?"

"In no way," Poindexter said, leaving the question of his actions in protecting the president still largely unresolved.