Former national security adviser John M. Poindexter told Congress' Iran-contra investigating committees in closed testimony last July that he did not attempt to read the key memo that outlined the diversion of Iran arms sales money to aid the Nicaraguan contras until after its discovery triggered his resignation, according to testimony released yesterday.

Poindexter's apparently passive reaction to what he called a "very strange piece of paper" raises new questions about the so-called diversion memo that has been at the center of the Iran-contra scandal. The validity of the document, which outlined a planned April 1986 arms sale to Iran and the use of $12 million from it to help the contras, has been questioned by some committee members and investigators.

Marine Lt. Col. Oliver L. North has testified he wrote this diversion memo and five or six others to Vice Adm. Poindexter for him to brief President Reagan. North said he thought he destroyed them all last November when the Iran-contra affair was exposed. It has never been explained how or why several versions of this one document survived the massive shredding that North and others said was undertaken in the White House.

Poindexter testified that he first heard of the existence of the document on Sunday Nov. 23, 1986, when North telephoned him at home to report on his meeting that day with Justice Department officials, including Attorney General Edwin Meese III. During the meeting the officials had confronted North with the document found in his National Security Council files the previous day and he confirmed that there had been a diversion.

In response to North's call, Poindexter said he was "getting pretty tired" and said, "Well, I'll see you in the office tomorrow." The next day, North recounted to him how the memo had been discovered and Poindexter said he responded, "I was surprised there was anything in writing on it" and asked North to send him a copy. "I don't remember the memo that you are referring to," Poindexter said he told North. Poindexter further testified that he did not actually ask to see the document until the next day, after he had decided to resign.

Poindexter added another element to the mystery surrounding the diversion memo when he told House and Senate investigators in the closed session that the version of the document that they showed him in July differed from the one he remembered seeing last November. The one he saw then, Poindexter said, did not indicate who was being asked to approve it. The version in the hands of the two committees specifically provides a place for presidential approval, which Reagan says was never given.

Poindexter acknowledged that in making the decision to approve the diversion of arms sales profits to the contras fighting the government of Nicaragua without Reagan's express authority, he could not defend himself against criminal charges by saying he was following orders.

"And you realize that you have therefore deprived yourself of the defense that you discussed it with the president and that the president approved this?" asked Senate chief counsel Arthur L. Liman.

"I understand that very well, Mr. Liman," Poindexter said.

In many respects, Poindexter's remarks in closed session were nearly identical to his testimony in the public hearings, but the depositions released yesterday included some additional points.

Poindexter disclosed that Reagan had written him a "departure letter" last Dec. 3, but did not describe the document. His lawyer, Richard Beckler, called it "not substantive."

Poindexter has repeatedly insisted that Reagan was not approving a trade of arms for Iran for American hostages being held in Lebanon. He was asked if he found such a trade to be distasteful.

"I frankly don't find that distasteful," he said. "I think that we live in a very imperfect world, a very dangerous world, and sometimes you don't have the best options or the ideal option, and you've got to do what's necessary."

Poindexter said he was not aware of any effort to get Vice President Bush to persuade Israel, through then-Prime Minister Shimon Peres, to accept responsibility for diversion of Iran arms-sales proceeds to the contras.

Committee sources said yesterday that there is some evidence that North may have discussed such a plan last November. Poindexter also said several times that Bush raised questions about the Israeli role in the arms deals with Iran, but the nature of these questions was blacked out of Poindexter's testimony, apparently for security reasons.

In a closing anecdote, Poindexter told committee investigators that North had told him that then-CIA Director William J. Casey "wanted to purchase an aircraft for the contras with his own private money. Col. North reported to me that he was a little surprised Director Casey had come to him, but that led us both to believe . . . that Director Casey was pretty aware of some of Col. North's activities" on behalf of the contras.

In May 1986, when Poindexter has described Reagan as looking for a way to take unilateral action to help the contras if Congress balked, North came up with the idea of the contras seizing Nicaraguan territory and for the United States to recognize them diplomatically, Poindexter said.

Asked whether the proposal went to the president, Poindexter said he could not recall but added it was considered serious.

Poindexter confirmed that Reagan gave his approval for a 1985 scheme involving agents of the Drug Enforcement Administration to use private money to buy freedom for hostages, directly violating administration policy against making concessions to terrorists.

Reagan has said through a spokesman that he remembered it but would not have approved a ransom. White House officials have said the money was to pay bribes for guards holding the hostages in Lebanon.

"This tended to be considered a bribe rather than ransom?" Poindexter was asked.

"That's exactly right," he said.

"You felt better when you called it that?"

"That's right."

Poindexter displayed throughout his remarks a suspicion of other White House officials whom he believed would leak information to the news media. He said, for example, that then-chief of staff Donald T. Regan was never told details of North's involvement in aiding the contras. "He talked to the press too much," Poindexter said. "I was afraid he'd make a slip."

The former national security adviser shed some new light on the ill-fated mission to Tehran led by former national security adviser Robert C. McFarlane in May 1986. Poindexter said he did not want to send North on an advance trip to Iran because he was afraid North might be taken hostage because he was a low-level official. Poindexter said he believed the Iranians would not dare capture McFarlane.