FOR DONALD Regan, the appearance before the select committees on the Iran-contra scandal was in the nature of a triumphal return.

Up in the Senate Caucus Room, the White House chief of staff who was forced out last February was sitting in the catbird seat.

Elegant in pinstripes and deeply tanned, Regan cheerfully, even boastfully, shredded Washington's image of him as an autocratic know-it-all.

The transplanted Wall Streeter, who once bragged, "Not a sparrow falls on the White House lawn without me knowing it," told the committee that as a matter of fact, he was odd man out when it came to the big stuff that was going on in the White House, particularly the crazed plots to befriend the Iranians, shake them down and send the boodle to the contras.

Oh, sure, he was at the meetings where the arms sales to the ayatollah were discussed. He was for it, then against it, then for it again. He took his cue from the boss. When the Iranians didn't come across with the hostages, he spoke of being "snookered" by "rug-merchants." When the initiative suddenly came to life again, he, apparently seeing the gleam in his leader's eye, decided they might as well give it another whirl.

He had no jurisdiction over John Poindexter and Oliver North. They reported to the president. He couldn't even get the documents they slipped to the president into "the White House paper chain."

It was hard to see from his performance on the stand why the Tower Commission had trashed him, why Republican legislators went down on their knees -- in his presence -- to beg Reagan to dump him. Here was a genial, witty fellow, with enough detachment to research the meaning of "fall guy" when he heard he had been chosen to be one. Here was a crisp Boston accent and a directness of expression that must have refreshed a president surrounded by sycophants who spoke in muddled pomposities.

A Boston Irishman who does not want to get even is a wonder, but Regan insisted that he is resigned to his resignation. He was once regarded as the source of the "chaos" in the post-disclosure White House. But like everyone else, he can see that his departure has not produced the miracle promised at the time -- the turn-around in public regard for the president. Regan's high spirits constituted an unspoken "I told you so."

After the fatiguing fog generated by two days of Attorney General Edwin Meese, he was indeed an east wind. "It isn't the spears in the breast I mind," he told Sen. Howell Heflin (D-Ala.), "it's the knives in the back that concern me."

To hear him tell it now, the sparrow-watcher spent much of his White House time steering clear of matters that were none of his business -- even matters that were of passionate concern to the president. Others in the White House were all but consumed by the Boland Amendment, which clipped their wings on contra aid. But not Donald Regan. Robert McFarlane said the National Security Council was bound by it; Poindexter and North said it was not -- though both told piously of hours spent trying to comply. Regan not only took no part in the debate, he had no views whatever about it. Being a good Reagan aide, he never asked any questions about it.

"I never looked into the legalities of the Boland Amendment," he told committee counsel Terry Smiljanich. "I never had the privilege of being a lawyer," he added slyly for the lawyer-packed committee, "and I didn't think I knew enough to be able to opine."

The only time he threw his weight around was after the arms plot had been uncovered and there were those in the White House, led by the president, who said that the real story would endanger the lives of the hostages. Regan told the president he must go public, must appoint a commission. Regan was all for disclosure of the diversion.

He had nothing against cover stories as such, he made clear. Sen. William Cohen (R-Maine) asked him why a cover story would preserve the lives of the hostages.

"I don't recall," said Regan negligently, "that we got to the point where we needed one."

What mattered to Regan? He gave a hint in an illuminating story he told about a day last November when George Shultz was franticly trying to warn the president he was dishing out disinformation. Regan plainly regarded the secretary of state as something of an old hen. Shultz' concern for the president, and the truth, seemed ill-timed to the chief of staff.

Margaret Thatcher was visiting and "I was part of the motorcade, part of the . . . . I had a seat on the helicopter. I didn't want to miss it."

The Wall Streeter had gone Washington. He wanted to be on the inside -- although not in on the secrets that can leave you begging for immunity and fearing the inside of a prison.

Mary McGrory is a Washington Post columnist.