President Reagan acknowledged last night in a nationally televised speech that he had traded U.S. arms for American hostages and said he accepted the "honest, convincing and highly critical" findings of the Tower special review board that investigated the Iran-contra affair.

"A few months ago I told the American people that I did not trade arms for hostages," Reagan said in a 13-minute speech from the Oval Office. "My heart and my best intentions still tell me that is true, but the facts and evidence tell me it is not."

This statement contradicted Reagan's previous statements about the Iran arms deals and came a week after the Tower report faulted both the initiative and Reagan's management of it.

While the president did not directly apologize for what he had done, as a number of his supporters and friends had urged him to do, he did say he took "full responsibility" for what had occurred. He also for the first time acknowledged that the initiative conflicted with his oft-declared policy of not bargaining with terrorists to win the freedom of hostages.

"As the Tower board reported, what began as a strategic opening to Iran deteriorated in its implementation into trading arms for hostages," Reagan said. "This runs counter to my own beliefs, to administration policy and to the original strategy we had in mind. There are reasons why it happened, but no excuses. It was a mistake."

In previous speeches and statements Reagan had denied that the Iran initiative ever became an arms-for-hostages swap. The closest he had come to admitting error was in a Dec. 6 radio address when he said, "it's obvious that the execution of these policies was flawed and mistakes were made."

Last night Reagan stuck to his original position that the Iran initiative began as an attempt to develop relations with potential successors to the government of Ayatollah Ruhollah Khomeini. He said it was "clear from the board's report, however, that I let my personal concern for the hostages spill over into the geopolitical strategy of reaching out to Iran. I asked so many questions about the hostages' welfare that I didn't ask enough about the specifics of the hostage plan."

Reagan then said to families of Americans still in captivity in Lebanon that "we have not given up" and "never will." But he coupled this with a warning previously issued by the State Department that Americans living in "dangerous areas must know that they're responsible for their own safety."

In a surprise announcement last Nov. 25, Attorney General Edwin Meese III said that some of the proceeds of the Iranian arms sales were diverted to the rebels, known as contras, opposing the Sandinista government of Nicaragua. Reagan said then and repeated yesterday that he did not know anything about this before it was brought to his attention a day earlier.

"I'm confident the truth will come out about this matter as well," Reagan said. "As I told the Tower board, I didn't know about any diversion of funds to the contras. But as president, I cannot escape responsibility."

In his meetings with the board headed by former senator John G. Tower (R-Tex.), the president first said that he had approved the original shipment of U.S. arms from Israel to Iran in advance of the shipment, as his former national security adviser, Robert C. McFarlane, had testified. Reagan subsequently changed his story to conform with the version given by his then-chief of staff, Donald T. Regan, which was that he gave approval after the arms were shipped.

In a final letter to the Tower board Reagan said he did not remember when he had approved the sale and said, "I'm afraid that I let myself be influenced by others' recollections, not my own."

Last night Reagan repeated the final version, incorporating it into a lament that "one thing still upsetting me, however, is that no one kept proper records of meetings or decisions." This was a point made in the Tower report and more forcefully in statements by Tower and former secretary of state Edmund S. Muskie, another member of the board, after the report was issued.

Reagan said that the absence of proper records "led to my failure to recollect whether I approved an arms shipment before or after the fact. I did approve it; I just can't say specifically when. Well, rest assured, there's plenty of record-keeping now going on at 1600 Pennsylvania Avenue."

The 76-year-old president, speaking in a muted tone, said he had learned that after a mistake, "you take your knocks, you learn your lessons, and then you move on." Reagan said, "the business of our country and our people must proceed," a lesson he had been hearing from Americans, U.S. allies "and, if we're reading the signals right, even from the Soviets."

This was a reference to a Soviet offer last weekend to proceed with an agreement to reduce intermediate-range nuclear missiles in Europe without making such an accord dependent on other arms pacts. Reagan said Tuesday that he welcomed the Soviet move.

While accepting criticisms of the Iran initiative that he previously rejected, Reagan defended his highly delegative "management style," which he called "a style that has worked successfuly for me during eight years as governor of California and for most of my presidency.

"The way I work is to identify the problem, find the right individuals to do the job, and then let them go to it," Reagan said. "I have found this invariably brings out the best in people. They seem to rise to their full capability, and in the long run you get more done."

But Reagan said that in managing the National Security Council staff, "let's face it, my style didn't match its previous track record. I have already begun correcting this."

The president then cited a number of previously announced actions taken by himself and by national security adviser Frank C. Carlucci, who replaced Vice Adm. John M. Poindexter after disclosure of the contra diversion. In particular, Reagan took credit for ordering the NSC to begin "a comprehensive review of all covert operations," an action he took more than three months ago.

"I expect a covert policy that if Americans saw it on the front page of their newspaper, they'd say, 'That makes sense.' "

Reagan also pointed with pride to the "accomplished and highly respected new team" he has brought into the White House, including Carlucci, chief of staff Howard H. Baker Jr. and William H. Webster, director of the Federal Bureau of Investigation, whom Reagan nominated Tuesday as director of central intelligence.

The president announced for the first time last night that he was appointing Tower to serve on the Foreign Intelligence Advisory Board "so that his knowledge of national security matters can be available to me on a continuing basis."

Reagan said he was "adopting in total" the Tower report's recommendations on how the NSC staff and process should operate.

"I am directing Mr. Carlucci to take the necessary steps to make that happen," Reagan said. "He will report back to me on further reforms that might be needed."

In his speech the president made no mention by name of Poindexter or Lt. Col. Oliver L. North, the NSC staff aide whom he fired after disclosure of the contra diversion but subsequently called "a national hero." However, the president distanced himself from these former aides, who have been the focus of investigations into the diversion of funds and who have refused to testify before congressional investigating committees on Fifth Amendment grounds.

"As angry as I may be about activities undertaken without my knowledge, I am still accountable for those activities," Reagan said. "As disappointed as I may be in some who served me, I am still the one who must answer to the American people for this behavior. And as personally distasteful as I find secret bank accounts and diverted funds, as the Navy would say, this happened on my watch."

White House officials said key sections of the speech were handwritten by the president from a draft largely prepared by Landon Parvin, a former speechwriter for the White House and for First Lady Nancy Reagan. The White House withheld release of the text until 7:30 p.m., after completion of the television networks' evening newscasts.