President Reagan transferred his full authority to Vice President Bush for nearly eight hours yesterday while undergoing intestinal surgery at Bethesda Naval Hospital.

Bush, who was on vacation at his seaside estate in Kennebunkport, Maine, abruptly returned to Washington just before the president's surgery began, out of "personal" concern for Reagan, officials said. The president delegated his powers to Bush as the vice president was flying back from Maine.

For all practical purposes, Reagan transferred full powers of the presidency to Bush during the surgery, including control over the armed forces. Reagan followed the procedures that the 25th Amendment outlines, but he expressed doubt about the applicability of the amendment, which establishes the legal method for transferring power if a president becomes incapable of performing his duties.

In effect, Bush had become the first "acting president" in the nation's history.

Reagan reclaimed his authority from the vice president by signing a letter at 7:22 p.m. at the hospital while he was recovering from surgery and still receiving painkillers.

Earlier, signing the documents giving power to Bush at 10:32 a.m. in his hospital room, Reagan quipped to his wife, Nancy, "I'm signing these letters, but you're still my first lady."

The powers were effectively transferred about an hour later, at 11:28 a.m., when Reagan went under anesthesia.

In letters to House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) and Senate President Pro Tempore Strom Thurmond (R-S.C.), Reagan said he did not believe the 25th Amendment applied to instances such as his temporary incapacitation by anesthesia during the surgery, but said he had nevertheless directed that Bush should exercise his powers temporarily.

The president made the decision after conferring yesterday with White House counsel Fred F. Fielding and chief of staff Donald T. Regan, spokesman Larry Speakes said. The White House also conducted extensive legal consultations last week with Attorney General Edwin Meese III, among others, and took into account a review of events that followed the attempted assassination of Reagan in 1981, Speakes said.

In that case, Bush and members of the Cabinet assembled at the White House Situation Room, but presidential aides decided not to invoke the 25th Amendment. Secretary of State Alexander M. Haig Jr. stirred an uproar in that episode by going before television cameras and declaring, "I am in control here, in the White House, pending return of the vice president," who had been traveling.

There was later some criticism of the decision not to invoke the 25th Amendment. White House aides hoped to avoid such criticism this time.

Reagan said in his letter yesterday that "I am mindful of the provisions" of Section 3 of the 25th Amendment "and of the uncertainties of its application to such brief and temporary periods of incapacity."

The section Reagan was referring to states that when the presdent informs congressional leaders he is unable to discharge his duties, the vice president shall become acting president.

"I do not believe that the drafters of this amendment intended its application to situations such as the instant one," Reagan said.

He added, "Nevertheless, consistent with my longstanding arrangement with Vice President George Bush, and not intending to set a precedent binding anyone privileged to hold this office in the future, I have determined and it is my intention and direction that Vice President George Bush shall discharge those powers and duties in my stead commencing with the administration of anesthesia to me in this instance."

A White House official said this language in the letter followed the procedures of the 25th Amendment, but that Reagan aides wanted to include a caveat to avoid setting precedents.

He said that officials wanted to avoid taking action that could require a future president to invoke the amendment for something as routine as dental surgery.

The amendment was ratified in 1967, four years after the assassination of President John F. Kennedy, which left President Lyndon B. Johnson, Kennedy's successor, without a vice president until 1965.

Officials said the temporary transfer of power to Bush would affect only a limited number of duties, and the vice president's spokesman, Marlin Fitzwater, said Bush took no actions under the authority yesterday.

Bush went to Maine Friday evening for a vacation and aides said then that he did not plan to return to Washington while Reagan was in surgery. But the vice president changed his mind early yesterday. Fitzwater said Bush made a "personal" decision to return.

He said Bush had also been in contact with chief of staff Regan about the possible transfer of power before leaving Maine.

Bush spent the afternoon at the vice president's Naval Observatory residence.

When Reagan awoke after the operation and aides asked if he was ready to take back his powers and return Bush to his No. 2 status, Reagan said, "Gimme a pen!" according to Speakes.

Even with the surgery, Reagan was engaged in presidential activity yesterday. He "read with interest" criticism of his recent concessions on possible budget cuts from Senate Majority Leader Robert J. Dole (R-Kan.), Speakes said.

Mrs. Reagan was joined at the hospital by her brother, Dr. Richard Davis, and watched the president through a window from an adjoining room as he was brought to the recovery room after surgery. A staff member telephoned the Reagan children, Speakes said.

White House officials set up temporary quarters at Bethesda, with chief of staff Regan arriving early in the day. Even during the surgery, Reagan was protected by a Secret Service agent.

Reagan remained in good humor before the surgery, Speakes said. He bid Nancy farewell before the operation with, "I love you," and told his doctors:

"After all you did yesterday, this ought to be a breeze.