It was Wednesday morning and the Cabinet Room was Ronald Reagan's rehearsal hall, as the president sat stage center awaiting the opening of this tryout for his first press conference in almost four months.

Deputy press secretary Larry Speakes had the first question.

"Mr. President, now that you have gotten your first round of budget cuts enacted and your second round has been proposed, do you think that these new cuts are attainable?"

Reagan replied with ease, as his aides recalled it, ticking off the pertinent facts, point by point, about how the cuts would be enacted because they were vital to the economic recovery, and how the markets and the public were feeling a new surge of optimism, and he cited some poll figures that showed just that. All in all, they noted approvingly, it was just about the way the suggested response had appeared in he 38 pages of questions and answers he had read the night before.

On Thursday, under the klieg lights of press conference reality, the first question, as expected, dealt with the economy. His answer, as rehearsed, hit all the main points.

The president had gone 15 weeks without holding a press conference--his last one, on June 16, having been a disaster by all accounts, including the private reviews of his own aides. Their reluctance to schedule another one only compounded the problem--making it appear that they feared that public discussions of substance were Reagan's own window of vulnerability.

But yesterday, Reagan's performance made it clear that this president, when thoroughly prepared and rehearsed, is still capable of finessing a press conference. In the face of only light-to-moderate interrogation from the press, Reagan seemed basically in command of his facts, in part because he was able to refer to a card that he had brought to the podium with him that contained a number of budget and welfare statistics.

Did Wall Street have confidence in his economic policies? He just happened to have a letter in his pocket. Did he have any second thoughts about the effects of his domestic program cuts? He produced a thick sheaf of regulations for the categorical grant programs that were being cut, and then showed several thin sheets of paper that he said contained all the regulations needed to administer the block grants that are taking their place.

And finally, the president had a convenient safety valve.

His advisers had taken the precaution of assigning the front block of seats on his right side to reporters they have taken to calling "known friendlies." He was told by his aides before the press conference that if he found himself in a sea of rough interrogation, he could turn to the right and call on one of a number of reporters his aides believed would be likely to ask less-pointed questions.

Seated on the aisle to the president's right were the Christian Science Monitor, RKO General, Copley newspapers, the Daily Oklahoman, and behind them were the McClatchy newspapers, the International Communication Agency, the Denver Post, and the Omaha World Herald.

As the press conference rolled on, ABC's Sam Donaldson asked whether Reagan was seeking "military superiority" over the Soviet Union. Reagan answered with his familiar phrase, saying that he was only seeking to close America's "window of vulnerability." When Donaldson tried to follow up on that, the president turned for the first time to his right and called on Christian Science Monitor bureau chief Godfrey Sperling Jr., who asked whether the nation could afford, in effect, both guns and butter.

Near the end of the conference, as the questioning persisted about whether Reagan was disappointed that the interest rates remained high, the president turned to the right once again and recognized RKO General Broadcasting's vice president, Clifford Evans. He asked Reagan what his thinking was about traveling to China.

Among the traditions of presidential press conferences is that at times presidents believe they are in too much trouble, at home or abroad, to hold them. In these times, reporters complain that the president is afraid to face "cross-examination." That is precisely what reporters have been saying recently at daily White House briefings with aides.

But another tradition of presidential press conferences is that reporters rarely cross-examine, and yesterday was no exception. When Reagan said, "Saudi Arabia, we will not permit to be an Iran," reporters did not cross-examine on what he meant or how the United States could justify intervention if the Saudi monarchy were overthrown by its subjects, without external interference.

Seven other questions were asked before Scripps-Howard's Ted Knap finally inquired how Reagan would prevent the overthrow of the Saudi monarchy. Reagan amplified on his pledge. But there the cross-examination stopped, as questioners turned to other subjects, once again.

At presidential press conferences, presidents are very much the commanders in chief. They hold them when it suits their needs, call on the reporters they wish to, and they have it in their power to turn all sessions of public questioning to their advantage.

Last June 16, Reagan seemed at sea almost every time he was asked a question on foreign policy: his responses were studded with I-really-can't-answers and I-don't-knows, and even the wink and the wave and the enaging, slightly off-center trademark smile had not saved him.

Yesterday, the Reagan who walked into the East Room was the beneficiary of a lengthy process of preparation and anticipation. It has become part of modern day White House tradition for presidents to undergo thorough briefings--at times even trial runs--prior to their press conferences. This was not the Reagan who had presented himself to reporters last June, having lazed over his briefing books and taken only a modest Q-and-A refresher course over lunch, just a couple of hours before his press conference.

For more than an hour Wednesday morning, the president's aides put their boss politely through his paces, firing questions on domestic policy. Their questions were strictly substantive, never argumentative nor laced with the sort of personal references to the lavish Reagan lifestyle that critics are fond of contrasting with his budget of austerity. After each answer by the president, his advisers offered suggestions, additions, deletions, changes of wording, and similar sorts of fine tuning.

Speakes and communications director David Gergen did the questioning. Budget director David A. Stockman and domestic policy chief Martin Anderson offered most of the suggestions of fine tuning, sources said.

Wednesday afternoon, the group convened again in the Cabinet Room for another hour-long session, this one on foreign policy. National security adviser Richard V. Allen was in attendance, in place of Anderson and Stockman.

At a noon lunch yesterday, the president and his aides talked it over one more time. There Speakes discussed the reporters' seating plan with the president. Rich Williamson, the president's liaison with state and local governments, entered to fill the president in on the status of the categorical and block grant programs. He had with him the sheets of regulations, old and new.

Reagan, not known as the great communicator for nothing, eyed the thick and thin stacks and smiled. "I'll take that," he said. Minutes before Reagan entered the hall, he had Speakes discreetly place the material on his press conference lectern--to be used as props for the question Reagan felt sure would be asked.

Yesterday afternoon, his press conference ended, the president left the East Room to the congratulations of his aides, who had been watching from near the doorway. This time, the president pronounced himself pleased.

"I always hate to leave those uplifted hands," he smiled, referring to reporters waiting to be recognized for questions. "We ought to have a three-hour press conference."