Former national security adviser John M. Poindexter, addressing a central mystery of the Iran-contra affair, told Congress yesterday he never informed President Reagan that proceeds from U.S.-Iran arms sales were used to support the Nicaraguan rebels, but added he believed that the president "would have approved the decision at the time if I had asked him."

Poindexter, a 50-year-old Navy rear admiral who met almost daily with Reagan from December 1985 to November 1986, told a hushed hearing room in the Rayburn House Office Building: "I made the decision. I felt that I had the authority to do it. I thought it was a good idea. I was convinced that the president would, in the end, think it was a good idea. But I did not want him to be associated with the decision."

In an ironic recasting of Harry S Truman's famous assertion that responsibility rests with the president, the former presidential adviser declared: "On this whole issue, you know, the buck stops here with me."

Under questioning from Arthur L. Liman, chief counsel for the Senate panel investigating the affair, Poindexter admitted that his motive in keeping the president ignorant was to provide Reagan "some future deniability" that would protect him from "political damage."

Poindexter also testified that last November, as the Iran-contra affair was unraveling, he destroyed a 1985 intelligence "finding" that Reagan signed, authorizing an arms-for-hostages deal. Poindexter said he "tore it up {and} put it in the burn basket behind my desk," adding that he had acted on his own authority "because I thought it was a significant political embarrassment and I wanted to protect {the president}."

Poindexter's testimony about the finding contradicted a series of statements by Reagan and White House officials about the long-missing document. The former national security adviser testified that Reagan signed the finding Dec. 5, 1985, which was as an after-the-fact authorization for a covert "hostage rescue" operation the previous month involving the sale of U.S.-made missiles to Iran.

The finding specified no goals other than securing the release of hostages in exchange for the arms, and the CIA's then general counsel, Stanley Sporkin, previously characterized it as a straight "arms-for-hostages" operation, which failed.

The president initially told the Tower commission last January that he had opposed that shipment and had demanded the return of the arms. He later changed that statement to say he couldn't remember any meeting or conversation about the shipment. In a July 1 statement to The Washington Post, a White House spokesman said there was no evidence that the intelligence finding ever went to the president, and thus he had never been asked if he had signed it.

Since last Nov. 13, when Reagan first acknowledged the arms sales initiatives, the president has insisted the secret transactions were primarily motivated by the broad strategic goal of opening relations with Iran rather than freeing hostages.

Poindexter made two other assertions that contradicted Reagan's publicly stated positions.

He said his "best recollection" was that he obtained the approval of the president for a nine-point proposal negotiated between American businessman Albert A. Hakim and Iranian officials that provided the basis for the last arms-for-hostage deal in October 1986.

The agreement included a promise of U.S. support for the removal of Iraqi President Saddam Hussein and the delivery of a plan for the Iranians to gain the release of 17 Lebanese terrorists held in a Kuwaiti prison -- two positions diametrically opposed to stated U.S. policies.

Reagan has denied ever approving, or even seeing, the nine-point plan, which was officially disowned by the United States after a copy was found in National Security Council files last December.

The second point of conflict came over testimony in which Poindexter said he had regularly briefed Reagan about activities in behalf of the contras, including those undertaken by North. The president told the Tower review board Jan. 26 he did not know the NSC staff engaged in helping the contras. Poindexter testified he had briefed the president about a clandestine airstrip built in Costa Rica by North's private resupply operation and also told Reagan that North was "instrumental" in keeping the contras supported.

The mild-mannered Poindexter, who occasionally puffed on a pipe, presented an image sharply different from the intense and often emotional North. The Navy rear admiral chose to appear before the committees in a business suit, rather than his uniform. In a pointed reference to North's appearance in a Marine uniform, Poindexter said, "I'm very proud of my uniform and I'm very proud of the United States Navy, but this is not a Navy issue."

Poindexter's categorical statement that he never told Reagan of the diversion came near the end of yesterday's dramatic morning session. But some senators said Poindexter's afternoon testimony raised questions of whether either Poindexter or North were credible on that key issue.

The questions arose over five memoranda that North said he sent to Poindexter in 1986. Each memo was prepared in connection with proposed arms transactions with Iran, and three of them eventually took place, North testified. Each memo laid out steps leading up to the transactions and, according to North, contained a paragraph describing how proceeds would be used to support the Nicaraguan rebels.

One draft memorandum of this kind was found in North's files last Nov. 22 during a "fact-finding" inquiry by Justice Department officials. It has sometimes been called the "smoking gun" memo because it called for the president to approve "the structure depicted above."

Poindexter testified initially yesterday that he did not recall seeing this particular memo until it was brought to his attention last Nov. 24 by the attorney general. And he said he had never seen any of the others, which North has testified he destroyed as part of a "cleanup" of his files suggested by the late CIA Director William J. Casey and known to Poindexter.

Later in the testimony, Poindexter said, "I don't recall getting any memos. Now, it appears that I may have gotten this memo at the time it was written."

Poindexter, showing the lengths to which he went to maintain Reagan's deniability, said he never discussed the diversion with anyone except North, whom he credited with proposing the idea in February 1986. "To my knowledge, the colonel and I were the only ones who knew about it," in the government, he said.

Poindexter said he never told Casey about the scheme because he knew the CIA director would have to testify before Congress, and he did not want to put Casey in a position that would require him to be "evasive." On the other hand, North has testified that he had consulted with Casey before he went to Poindexter with the idea for it. Assuming all the testimony has been true, Poindexter and Casey kept their knowledge from each other.

At the same time, Poindexter said he was not familiar with a Casey plan to operate an offshore "full-service" covert entity, financed with non-government funds, as North had testified last week, although he said he did recall some discussions about the "feasibility of doing some other things."

In addition to presenting the committees with conflicts with North's testimony, Poindexter's statements also pose problems for Secretary of State George P. Shultz, Attorney General Edwin Meese III and former national security adviser Robert C. McFarlane.

On Tuesday, McFarlane told the committees he had not authorized North's activities on behalf of the contras in 1984-85. Poindexter, who was McFarlane's deputy in this period, said "it was my understanding Mr. McFarlane had authorized the activities that had taken place prior to December 1985."

He also differed with McFarlane's testimony when he said, "I don't believe the Boland Amendment ever applied to the president or the National Security Council staff."

Poindexter's testimony set the stage for the committees to question Shultz about the extent of his knowledge about the details of the 1986 U.S.-Iran program. The secretary has frequently stated that because he was a strong opponent of the initiative he was denied information and was misled by Poindexter in May 1986.

Shultz has said Poindexter told him then that the operation was over, but Poindexter testified he did not recall making it that definitive. He also said the secretary of state knew that arms had been shipped to Iran in 1986 because Poindexter talked about it at a "family group lunch," a regular gathering of Casey, Shultz, Poindexter and Defense Secretary Caspar W. Weinberger in the White House family dining room.

Of Shultz, Poindexter said: "He indicated that he didn't particularly want to know the details" of the Iranian arms deals because "he also understood that the president wanted to go ahead with it" despite the secretary's opposition.

"He said, just in effect, tell me what I need to know," Poindexter testified.

Poindexter, who went to work at the NSC in 1981, described himself as more knowledgeable about Reagan's views than any other member of the White House staff during his final year. He provided a number of personal observations about the president's style and the foibles of the NSC staff.

Reagan, he said, customarily read every document put before him and occasionally signed drafts of documents that met his approval before they were in final form.

The president "is not a man for great detail," Poindexter said. But when that brought a ripple of laughter, he added, "I don't mean that in any funny way. A president shouldn't be involved in detail."

Poindexter said it was his policy not to bother the president with details of "implementation." He included in that category the diversion -- which he said was only implementing the president's desire to keep the contras going. Poindexter also said Reagan never asked which foreign countries were providing funds to help the rebels.

Reagan was also a man of action who, after reading a book about the Israeli raid to rescue hostages in Entebbe, asked Poindexter in regard to helping the contras: "Isn't there something I can do unilaterally?"

He described North as one of the most capable officers who had ever worked for him and said the young Marine would have been promoted to the position of assistant to the president were it not for Poindexter's desire for North to keep a "low profile" so he could perform his covert operations.

His one criticism of North was that he had failed to master the technique of destroying his computer messages in the White House. "My policy was to erase them, and I apparently did it the right way, and I don't think Col. North did it the right way," he said.

Although Poindexter's testimony is based on long, private depositions with committee lawyers, thus reducing the chance of surprises, the witness nonetheless came out with several remarks yesterday that appeared to be slips.

At the end of a series of questions on aid to the contras, he was asked by Liman, "Did you see, as one of the benefits of the Iran arms sales, that money would be generated for the contras?"

Poindexter, who had repeatedly made the point that the president knew nothing about that proposal replied: "No, that -- that did not play a single part in my mind or the president's mind in deciding to go ahead with the Iran project."

Liman went on with another question, but Poindexter's attorney Richard W. Beckler, abruptly interrupted with one of his numerous objections of the day. Beckler later said Poindexter is a target of the investigation of independent counsel Lawrence E. Walsh.

During another series of questions about the drafting and evolution of a January 1986 intelligence finding in which Meese participated, Poindexter implied at one point that Meese had knowledge of several possibly illegal 1985 transfers of U.S.-made arms from Israel to Iran.

"My understanding of his {Meese's} position was that although we could continue to do it the way it had been done earlier, that it was easier to support and clearer if we did it direct . . . . " he said.

Asked about this yesterday, a spokesman for Meese reiterated that the attorney general had no knowledge of the 1985 activities until November 1986.

In his questioning, Liman focused on Meese as the person who was primarily responsible for changing the method of transferring arms to Iran by selling them direct instead of adopting another idea, to permit the Israelis to ship the weapons. The new method of selling direct, said Poindexter, removed the necessity of informing Congress and made it possible to limit the number of people in the executive branch who knew.

One of the most dramatic episodes described yesterday by Poindexter occurred last Nov. 21, the day he said he made a decision on his own to get rid of the controversial intelligence finding authorizing the CIA's assistance to the Israelis in their arms shipment to Iran one year earlier.

Meese called him that day to arrange for Justice Department officials to examine the Iran documents at the NSC. Later, Poindexter asked a staff member to bring the findings from a safe. This was two weeks after the first U.S. news reports of the Iranian arms sales and two days after a presidential press conference in which Reagan denied U.S. awareness of the 1985 Israeli shipments and issue a correction moments after the press conference ended.

The president, Poindexter said, "was being beaten about the head and shoulders that the whole Iran projects was just an arms for hostage deal." He said "this finding, unfortunately, gave that same impression," and "I frankly didn't see any need for it at the time."

Questioned by Liman, Poindexter said it was the responsibility of the staff to make certain that the president "not be put in a position that can be politically embarrassing."

North had testified that, on the same day, he had talked to Poindexter and reassured him that there were no records left of the diversion and proceeded to destroy additional documents related to the contra operation with the assistance of his secretary.