Donald T. Regan admits to a bit of nostalgia as the White House girds itself for the trip to the Bonn summit next week. As Treasury secretary, he, along with Secretary of State George Shultz, sat at the president's elbow at earlier summits.

Now, having switched jobs with James A. Baker III as President Reagan's chief of staff, Regan will be going to Bonn, but admits he will be outside of the conference room, looking in. Nonetheless, irrepressible good humor wells up in the 66-year-old Irishman: "At least," he says with a grin, "I won't have to do any of the official briefings -- I can just do the leaking!"

In a long interview, Regan made clear that he relishes his new role as chief of staff for the president, except for the calamitous decision to visit the Bitburg military cemetery in West Germany on May 5, after the Bonn economic summit. "That was a downer," Regan admits.

He won't talk about it, but clearly the Bitburg mess grates on him: He knows that, everywhere in Washington, people whisper that Baker would have quickly found a way to shed the embarrassment of the proposed visit to a burial site that includes graves of Nazi SS men.

With relief, Regan turns to the more familiar ground of the economy, which he says still occupies much of his time, although, in the restructuring of the Cabinet councils that he ordered, Baker has the lead role in domestic and international economic affairs.

An IBM computer sits atop his desk for the primary purpose of calling up the latest information on stocks, bonds and other economic information, including the dollar exchange rate.

"I keep a very sharp eye on" economic policy, he said. "There is no policy, economic or domestic, that somehow or other doesn't pass through the West Wing before it gets to the president. We're engaged in all the policy discussions. So while I act as a manager, I also act as well in the role of helping to shape policy."

Regan in recent weeks has become concerned about stagnation in the economy and the possibility of a recession. He said in the interview that the first-quarter growth rate of only 1.3 percent was a bad sign, especially "because the third and fourth quarters of '84 were not that great." Over the nine-month period, the economy has been growing at an annual rate of about only 2.5 percent, a pace that almost guarantees a rise in unemployment.

The dismal first-quarter picture came as a shock to the administration. It's especially embarrassing to be on the way to the Bonn summit with the economy threatening to go into a tailspin: In London last year, and at Williamsburg in 1983, President Reagan was able to brag about American economic growth -- and claim that the United States could "grow" its way out of the deficit.

Now, with the economy tilting down and the deficit tilting up, the administration desperately needs a new economic theme. And in searching for one, it has cast aside its earlier insistence that the huge budget deficits have nothing to do with high interest rates. This is the way Regan put it:

"We've got to cut this deficit so we don't have to borrow as much over the next few years, and take that amount of capital out of either the domestic supply or the international supply of funds. We'd rather leave it in the pool for the private sector."

That's not much different from what the Democrats and foreign critics have been arguing for the past two years: A lower deficit "We've got to cut this deficit so we don't have to borrow as much over the next few years, and take that amount of capital out of either the domestic supply or the international supply of funds. We'd rather leave it in the pool for the private sector." -- Donald Regan will allow the Federal Reserve to follow a more lenient monetary policy, dropping interest rates and, hence, the international value of the dollar. That, it turn, could help make American industry more competitive.

"We would rather have more fiscal discipline and therefore make it easier on the monetary authorities to provide funds to get interest rates down," Regan said. That's a 180-degree turn from what he was saying as Treasury secretary. The party "line" then was that there was no certifiable connection between deficits and interest rates.

One of the main proponents of the no-connection theme was Treasury Undersecretary Beryl Sprinkel, now chairman of Council of Economic Advisers. Noting that Baker is the No. 1 man on economic issues, Regan volunteered: "And I didn't put my own man Beryl Sprinkel in charge of it, notice that!"

Regan has revamped the White House staff operation so that he's the boss, not dividing responsibility, as Baker did, with Michael Deaver and Ed Meese. "It's being written about as unique, but actually the three guys was a unique arrangement," Regan said. He sees his own relationship to Reagan as analagous to more traditional ones such as Hamilton Jordan's role for President Carter, Donald Rumsfeld's for President Ford or H. R. Haldeman's for President Nixon.

As such, Regan will be taking the credit for White House efficiencies, and the rap for failures, as he inevitably will for the Bitburg mess, even though it apparently got under way before he took over. It's easy to see why he's straining for a resurgence of the economy.