At 7:15 last Friday morning, a diminutive, stooped, white-haired man of 81 reported to the Office of the Secretary of Defense on the third floor of the Pentagon to hear the historic decision President Reagan had made the night before.

As gently as they could, Defense Secretary Caspar W. Weinberger and Navy Secretary John F. Lehman Jr. told their early-morning visitor that the president had decided to end an era of naval history and replace him with a younger man.

The visitor was Hyman George Rickover, son of a Jewish tailor who had left Russian Poland with $50, set up a shop on New York's East Side and then sent for Hyman and the rest of his family in 1904.

By 1922, Rickover had beaten the long odds facing a poor Jewish boy and graduated from the Naval Academy. No one dreamed this loner out of Chicago would get very far in the often anti-Semitic Navy of those days, much less make full admiral and, in his own way, change the world for all time.

But Rickover did indeed achieve those things in the subsequent 59 years, again against long odds. Weinberger and Lehman thanked Rickover for all this in their 45-minute meeting but told him he would not be extended on active duty again; that it was time to let someone else run the Navy's nuclear propulsion program and the Energy Department's reactor office.

The commander-in-chief was ordering him into retirement, whether he liked it or not.

The day before, Rickover, in a meeting in the office of Sen. Strom Thurmond (R-S.C.), had made it clear to a group of his congressional supporters that he did not want to go; that he wanted his job extended again when his latest reprieve from retirement ran out on Jan. 1.

Weeks before, Rickover had been telling friends that there were a number of people out to get him. But Congress had saved him from career extinction many times before. Could it again?

"We were meeting again on Friday in Thurmond's office to plan a course of action," recounted Sen. Henry M. Jackson (D-Wash.), probably Rickover's strongest ally in Congress. "But before the meeting was over, they just up and did it."

The word was put out at the White House and then the Pentagon that Rickover was going to be retired.

"We wanted to talk to the president to work out a transition," Jackson said in complaining of Reagan's action on Rickover. "But the decision was just ad-libbed."

Jackson, Thurmond, Sen. John W. Warner (R-Va.), Chairman Melvin R. Price (D-Ill.) of the House Armed Services Committee, Reps. Samuel S. Stratton (D-N.Y.), Frank Horton (R-N.Y.) and Floyd Spence (R-S.C.) were in the congressional blockhouse planning a Rickover rescue mission when they were hit by Reagan's preemptive strike. Will they launch a counterattack?

"These things are up to the president," Jackson replied when asked that question in a telephone interview yesterday. "You can't change the Constitution. We haven't decided what to do."

With no congressional help immediately in sight, Rickover is down to life size as a naval officer as far as following orders from his president. And White House advisers who told Reagan that Rickover could be retired without much fuss apparently were right.

Moves toward retiring the controversial admiral started shortly after Reagan took office. Months ago, Weinberger aides passed the word that the secretary favored a change. Lehman made a political damage assessment and concluded it would be slight, given Rickover's eroded power base in Congress.

Unhappy ship builders, particularly the Electric Boat Division of General Dynamics, which builds the Trident missile submarine and the 688-class attack submarine, nudged the anti-Rickover effort along whenever they could, including at the White House.

Rickover's fabled intelligence network threading through Congress, the Pentagon and much of the executive branch sent back warnings to his Crystal City office. The admiral frequently called up old friends and complained of a conspiracy "to get me."

He claimed unfavorable newspaper articles and a critical book about him were part of it. But his supporters did not rally until it was too late. Without talking to Rickover first, Reagan approved the retirement scheme Thursday night and had the Pentagon announce it in a news release on Friday.

As a consolation prize, Reagan has asked Rickover to come to the White House next year to work the same magic on the civilian nuclear power program as he did on the Navy one.

Will Rickover serve under the same president who ended his 33-year reign as king of the nuclear Navy? "No way," said one of his friends who should know.

Rickover has refused to say what, if anything, he will do for an encore now that he has won the plaudits of every president--including Reagan--since Harry Truman spoke at the keel-laying of the first nuclear-powered submarine, the Nautilus, on June 14, 1952.

"This ship will be something new in the world," Truman said back then. It was. And so was Rickover.