One of the time-honored perks of secretaries of state and defense is the right to commandeer an Air Force transport plane and take off for foreign shores with a working staff, a duplicating machine and accompanying journalists. That way they meet their opposite numbers in other governments, sell a president's program and, incidentally, make headlines.

Henry Kissinger was the past master of this art of jet age public relations. But lately, secretaries of the Treasury have caught on, and Donald T. Regan is no exception. He warmed up with a brief trip to China, and now is barnstorming Europe on his first full-scale venture, bouncing from capital to capital.

The Regan swing is a blend of official meetings--the OECD in Paris, the IMF-World Bank in Helsinki--and an unofficial but open sales pitch for Reaganomics before private audiences.

The message of this affable Irishman and former head of the biggest stock brokerage house in America is borrowed from its advertising slogan: he's "bullish on America."

Aboard his plane en route here, while autographing a pile of $1 bills as souvenirs, Regan told me: "I can meet these fellows and see what's on their minds. I can hear their complaints, or maybe rebut what they have to say, and report back to the president."

Regan is good at it. He defused much of the hostility in Paris over high interest rates by pointing out that lower rates would be "no panacea for the rest of the world's ills." But there remains anti-American feeling in Europe, palpable among European journalists.

With a touch of candor and a ready wit, Regan often disarms the questioners. He's relaxed and approachable--a far cry from the typical stuffy and formal European finance minister.

An ex-Marine, he enjoys hearing and telling a good joke--an attribute that also appeals to his boss. If he doesn't agree with what he considers a loaded question, he'll challenge the premise.

Yes, he will admit, things haven't gone as planned in the United States--whole American industries, such as automobiles, are "on their knees," or like the building industry "flat on its back." But there is no easy way out: we hate high interest rates as much as you do; we're doing the best we can to get them down. And so on.

When a reporter in Paris asks if President Reagan will not find himself "isolated" at next month's economic summit at Versailles, Regan snaps back without breaking stride: "Of course not, we'll all be there with him." Then, enjoying the joke with others, he lets the laughter subside and gets serious. The president, he assures us, will share Europe's chief worry: rising unemployment.

Walking into a press conference here straight from the airport, Regan was hit with the question he has faced all through this trip: "When will interest rates come down?" His answer: "I spent 35 years in Wall Street ducking that question, and I'm not going to start answering it now." But he can't resist adding a promise that rates "should be lower by the end of the year."

When pressed to defend the American decision not to intervene in exchange markets to stabilize the dollar, Regan snaps: "Our response to that is: 'We're from Missouri.' We don't believe that intervention actually works."

Regan successfully fended off an attempt by the other OECD ministers to get a demand for intervention into the OECD communiqu,e--and he's proud of that diplomatic finesse.

But Europeans, while they appear to like the Regan warmth and friendly manner, aren't sure they like the hard line underneath. "The tendency is for the secretary and other Americans to say, 'our policies are right, and it doesn't make any difference what you say,' " sighs a veteran bureaucrat in Paris. Reagan believes that America gets it way because of "the force of our logic." Rebuts the bureaucrat:

"To us it's frustrating, but we nevertheless hope the Americans succeed: If they don't get their deficits down in 1983 and 1984, Europe will be in an unholy mess."