Promptly at 7:30 a.m. last Tuesday, Secretary of State George P. Shultz strode into a small private dining room at the Treasury Department and said to Treasury Secretary Donald T. Regan:

"What the hell are you saying now -- did you change your mind?"

Regan, relating this incident to a visiting reporter the next day, said that Shultz was referring to a story that morning in The Washington Post quoting Regan on the potentially serious effects of a further drop in oil prices on the Third World debtor countries.

"I told him, "hell, no!"" Regan continued. "Sure, if the price of oil takes a drop, it will hurt Nigeria, Venezuela, Indonesia, Ecuador and so forth, and sure, it'll make for choppy waters temporarily. But the other half of that story is an oil price drop will eventually help everybody in the world economy, consumers and oil-importing nations."

Shultz" very presence in Regan's dining room last Tuesday morning was a symbol of the secretary of State's broad involvement in all economic issues. With Regan as chairman, they meet as a group of six -- an expansion of the old "quadriad" on economic policy. In addition to Regan and Shultz, it includes budget chief David A. Stockman, Commerce Secretary Malcolm Baldrige, White House assistant Ed Harper, and Economic Council Chairman Martin S. Feldstein.

Harper is there as a sort of monitor for the White House staff. But Shultz is there because he asked to be included. And given his expertise and strong continuing interest in economic affairs, his relationship with Regan could have been a difficult one. But the two men have worked out an arrangement that Regan says keeps them "from treading on each other's toes.

"I recognize -- what is the buzzword? -- his "area of competence" in the field of East-West trade relations. He recognizes my "area of competence" in other economic issues, particularly such things as [exchange rate] intervention or the monetary side -- or indeed, [issues related to] economic recovery and all that."

In addition to being close to Shultz ideologically, Regan is a good personal friend. The Shultzes were the Regans' Virginia house guests last year, until they got settled in their own home.

Nonetheless, Shultz doesn't hesitate to speak out on economic issues, or to give the president his views on budget and tax matters.

Regan doesn't feel at all pressured or uncomfortable. He survived an internal leadership contest with Stockman earlier in the administration in good shape. More important, he has developed what White House staffers describe as a "buddy-buddy" relationship with the president, even though his relationship with those same staffers is not close, merely friendly.

Regan's good personal standing with Reagan -- they enjoy swapping jokes -- has enabled the Treasury boss to press Reagan on the issue of closing the budget deficit gap, while remaining a team player publicly articulating the administration's party line at any given moment.

This may have cost him some points among his old Wall Street pals, who think he bounces up and down too easily on economic issues. He's been burned, occasionally, by going public with an optimistic forecast made clearly for political reasons. But Regan, former boss at Merrill Lynch, still thinks in Street language, and is a good transmission belt for the president on what businessmen and bankers are saying.

Like his old colleagues, Regan is genuinely worried about huge budget deficits, and is trying to convince the president that some new tack must be tried to deal with the torrent of red ink.

"Myself, I'm quite concerned about the budget deficit in the "out years' because of the psychological impact," Regan said reflectively. "I do not think that we can continually offer to the money markets $200 billion deficits or thereabouts and expect that they will infer anything except inflation and higher rates of interest and you're apt to generate and make this a self-fulfilling prophecy."

Regan is also influenced by the shrill cries of pain over the high interest rates and overvalued dollar his fellow finance ministers all over the world ascribe to the deficit. He has been getting a bellyful of this argument on recent trips to Europe and Asia, and knows it will be repeated next week at Williamsburg.

He's not only sympathetic to their economic complaints, but much more aware than he was two years ago of the interrelation of politics and economics:

"This year, the politics of economics is going to be very important to all of these nations and on their mind. Unemployment is a politically damaging thing to any politician. How to get unemployment down is one of their main concerns as well as how to "rev up" business -- where to get these new jobs.

"Now, they have been looking at their traditional industries, the smokestack industries, and they're going to have to get to the realization that that's not where the new jobs are. The United States has at least found the key that it's in light industry, service industries, things of that nature. Japan, of course, has been doing it by pushing export markets."

Regan and Shultz have a joint objective of promoting the idea of economic growth at the Williamsburg summit, along with a reversal of protectionism. It is a more broad-gauged goal than that of the White House California Mafia, who more simply hope that President Reagan will not stumble, but come off looking like a successful leader.

What the president has going for him is the happy coincidence of brighter American economic indicators, sufficiently strong for the moment to paper over the more serious potential impact of a growing protectionist war and Third World debt problems.

Says loyalist Regan: "I think Ronald Reagan will come out of this [summit] looking good if he's able to assure people and convince his counterparts that his program is succeeding and that he's a man to be listened to."