Newly installed White House chief of staff Donald T. Regan made his debut yesterday before the White House press corps and promptly encountered hostile questioning about his choice of conservative commentator Patrick J. Buchanan as White House director of communications.

Regan gave a spirited defense of Buchanan, a combative speech writer in the Nixon administration who as a columnist frequently has targeted moderates in the Reagan administration.

Regan said he expects Buchanan to present his ideological views forcefully in White House policy discussions but to represent the president's decision loyally whether or not he agreed with it.

"I reminded Pat of an old phrase I think most of you will recall of accepting the king's shilling," Regan said. "And when you accept the king's shilling, you sign aboard. . . . In coming with this administration, he agrees that he will support the administration's final position whenever he reflects a position to the public, to the media and in other public utterances."

The questioning of Regan in the White House briefing room reflected distrust of the new communications director based on Buchanan's persistent criticism of the media.

Although Regan used the occasion to announce appointment of Buchanan and two other assistants to the president, legislative strategy coordinator Max L. Friedersdorf and political and governmental affairs strategist Edward J. Rollins, almost all of the attention focused on Buchanan.

After Regan had said he did not think that Buchanan "sees the press as an enemy or the media as a whole as an enemy," ABC White House correspondent Sam Donaldson read Regan a portion of a column written last year by Buchanan.

"An ideological bulwark of the Democratic Party, polemical and publicity arm of American liberalism, the big media are the strategic reserve of the Walter F. Mondale presidential campaign," the column said.

"He didn't say some reporters, some media," Donaldson continued. "In all of his pieces, Mr. Regan, he lumps under the name 'big media' every one of us."

Regan tried to turn the questioning aside, saying, "Well, that may mean he's a good communicator and it may not, but we'll have to see."

Asked how Buchanan could represent an administration fairly after four years of attacking its moderate wing, Regan said the president believes "that he's best served when he has a myriad of opinions around him, that he shouldn't have just monolithic opinion being offered to him or, indeed, have only 'yes' men around him."

While reactions indicated that Regan apparently made few converts to his view of Buchanan, he impressed several in the room with a breezy, confident style punctuated with flashes of humor.

Asked what role Buchanan is expected to play, Regan asked his questioner if he remembered "the old Dave Gergen role," referring to David R. Gergen, the moderate who served as Reagan's communications director for the first three years of his presidency and who often was under fire from conservatives.

Reporters responded with a wave of skeptical laughter.

"You haven't allowed me to finish," Regan said. "It'll be changed from that."

Regan also announced that John A. Svahn will remain in charge of the Office of Policy Development, that James S. Brady will continue as press secretary and that Larry Speakes will remain as principal deputy press secretary with responsibilities as day-to-day spokesman for the administration.

Regan said Speakes would report directly to him and to the president rather than to Buchanan.

Friedersdorf and Rollins have forged reputations as pragmatic advocates of Republican policies who have been more conciliatory than confrontational in dealing with Congress and the press.

Friedersdorf, vice president of Pepsico Inc. since 1983, won the respect of Republicans and Democrats alike as White House congressional liaison during the Nixon and Ford administrations.

Rollins, a former Republican legislative aide in California, won wide respect in his party as national director of the president's reelection campaign, Reagan-Bush '84.