Six months ago he was running a mini-empire at the nation's largest securities and brokerage firm. Now he has not one but several bosses, and his time is not his own.

Although Donald T. Regan's new job as treasury secretary is in a way a promotion from his old one, the former chairman of Merrill Lynch exercises less direct power and commands less automatic respect than he did in his previous incarnation -- and that takes some getting used to.

"When I was head of Merrill Lynch," he said recently, "I set my calendar, and everyone else marched to that time."

Now he is summoned before congressional committees -- at their convenience, not his. The man who is accustomed to swift obedience finds himself instead subjected to challenging cross-examination. It has been obvious at times during his seven weeks in office that he does not always like it.

When a young congressman last month described the administration's forecasts as "hallucinogenic," Regan shot back, "I resent that." A near shouting match developed.

Mostly Regan has kept his temper with needling congressmen and journalists alike. And while his combative nature could dent his effectiveness as a salesman to Congress, it has sometimes served him well in Washington. In a recent early-morning press briefing, laid on as part of the selling of the economic program, it provided a quick answer to reported congressional jibes: "Those Democrats on the Joint Economic Committee didn't do so well either."

But quick answers do not always fill the bill.

There is no doubt that Regan is an extremely able man. He masterminded the explosive growth in Merrill Lynch, now three times bigger than its next-largest competitor. He was a Wall Street innovator whose ideas have paid off enormously.

But so far he does not seem to have settled enough into his Washington role to have gotten a firm grasp on the reins of economic policy-making.

He was designated by the president early on as the administration's chief economic spokesman, but administration officials agree that he was not at center stage in the planning of the president's economic program. He was, they say, surprisingly uninvolved in heated administration discussions over just how optimistic the Reagan team should be about the likely effects of its economic program.

Regan's newness to Washington, to the intricacies of economics and perhaps to the Reagan camp, were probably all factors. Budget Director David A. Stockman, who had taken an early lead in developing the program, went on calling the shots after Regan's appointment, some officials say.

In a way, this was not surprising. Last November, while Stockman was drawing up his blueprints, Regan wasn't thinking about how to run the economy. He was running Merrill Lynch.

But the 62-year-old ex-Marine gives the impression that, given a moment to catch his breath and assemble a staff, he will dominate. He told reporters in January that he expected Stockman, whom he characterized as a "very clever young man," would "return to a more traditional role" of managing the budget while Regan took charge of overall economic policy.

That still could happen. Insiders scoff at the idea that Regan and Stockman are fighting for position in the economic field, however. If they are, both are too controlled and private to let it show, one source said. Perhaps just as important, Stockman does not appear interested in personal politicking, relying on his conviction and mass of facts to press his case, an OMB official said recently.

That conviction and mastery of detail has also helped the budget director on Capitol Hill, where he responds fluently and effectively to most congressional questions even when what he says flies in the face of much economic wisdom.

Regan has shown a much less sure grip on the rationale behind the administration's program. He has on several occasions appeared stumped for an answer when challenged on some detailed but crucial aspects of the administration's policy.

"What share should investment take of the nation's total output?" one congressman asked the treasury secretary last week, plainly skeptical about the administration's claim that the next few years will see a sharp increase in the rate of business investments. He did not get a straight answer until he gave Regan a clue, telling him investment's current share of national output.

Earlier, the treasury secretary told reporters that one-third, possibly one-half, of all the proposed tax cuts for individuals would be put into savings -- an astonishing figure. Treasury officials later retracted the claim, which seemed to be the result of confusion with another aspect of savings.

Regan is quick to reject suggestions that he is not really a convicted "supply-sider."

"I said yes [to the job of treasury secretary]. That means that I agree with the president's policy," he argues. But while he may agree with it, he certainly did not help to formulate it, and on occasion has seemed as if he is giving stock administration answers to critics rather than reasoning of his own.

Regan, a graduate in economics (and English) from Harvard, rarely commented on the economy while on Wall Street. Indeed he did not speak much in public at all, and was known as a private man, close to his family.

One position he was apparently proud to hold outside the financial world was on a rules advisory committee for the Professional Golf Association. He is a keen golfer, fond of golfing analogies, and friends say his style on the golf course fits in with his public persona. He is an avid, aggressive golfer, says one friend, adding, "by aggressive, I mean he likes to win."

He plays his heart out in a common golfing game called $1 Nassau, the friend continued, where the top win is $3. And unlike many weekend golfers, he's a stickler for the rules.

Treasury staff members have already seen that he does not suffer fools gladly. When he first came to Washington, he made no secret of his contempt for Treasury's foreecasts and those producing them, telling reporters and members of Congress that he intended to improve the reliability of his department's numbers.

Once, in answer to a senator's request for some information that could be found, the senator said, "by any competent man in your department," the secretary replied, "That's if I can find one."

But more recently he has been generous with his praise for the hard work of his staff, admitting that he was "surprised at the hours people work [in government] for the recompense they get. . . . It really shocks a guy coming from Wall Street."

The Wall Street impatience with Washington may pass in the coming months, as Regan finds his position in the administration.

One key will be how the Reagan administration's experiment with Cabinet government goes. The treasury secretary is chairing the new Cabinet council on economics, and obviously sees this as a means of gaining control over economics policy.

"I shall control it," Regan said of the economics council, explaining that it would therefore not be just one more demand on his time. Stockman sits on this council, but does not chair one of his own.

If the system of councils works, and if he is backed up by the White House, Regan is bound to take a more active role in formulating policy in the future, according to one Republican closely involved in the transition. Regan may feel more comfortable chairing such a committee, through which all important aspects of economic policy are expected to flow, than he is testifying on the Hill or discussing the minutiae of economic forecasting.

Few doubt that the forceful man, who was described as something of a martinet in Merrill Lynch and calls himself a "good soldier" in the administration, can run his department. But during the transition period he is suffering from the woes of many an executive who deserted the boardroom for a federal office.

"I was known as a manager and financial innovator in my previous role," he said last week. "Now I am doing neither."