Imagine yourself in a school multi- purpose room. It is 9:30 on a Saturday morning, with an autumn drizzle outside. About 120 men and women are sitting in a semicircle of folding chairs. All live in Northern Virginia, but their ages and occupations are as diverse as the mixture you would find in any nearby suburban shopping mall.

One man takes the podium and argues in a three-minute speech that the members of the convention who wrote the original Constitution fully intended that the people themselves would change that Constitution, as needed, by the same mechanism -- a constitutional convention. "To deny that option," he says, "is to distrust the people."

The following speaker says that, of course, that method of amendment is available, but it should not be used "to deal with transient policy issues . . . but only in times of supreme national catharsis." The debate swings back and forth between proponents and critics of amendment by constitutional convention, until the presiding officer opens the floor to all the "delegates." Their comments and arguments fill the air, much as they did in Philadelphia when Article V -- the amending clause -- was first debated.

What is going on here? It is the Jefferson Meeting -- a citizens' forum that takes the Constitution of the United States off the library shelf and makes it, once again, the center of attention for Americans concerned about the health of their republic. They have come together for a day of intense debate, designed to stimulate their thinking on the question whether the Constitution, in these altered times, is well adapted to serve its original ideals.

I was asked to preside at part of the Jefferson Meeting in Arlington, and I have rarely been part of a more stimulating and enjoyable gathering. It's my hope that many more communities and schools can discover the excitement of this experience.

The project was launched two years ago by a fine retired journalist, Charles L. Bartlett. Its aim, says Alice O'Connor, the first director, is simply to "promote discussion of the first principles of our government and provide a more critical appreciation of the Constitution." It is not an advocacy group or a forum for debating school prayer, abortion or balanced-budget amendments.

The first Jefferson meeting was held in March 1984 in Williamsburg, Va., and the format has been tested in several other sites. Now foundation director Dick Merriman says he is ready to help groups around the country with guides for organizing community and school forums and compact study guides for six issues. (He can be reached at 1529 18th St. N.W., Washington, D.C. 20036. The phone is 234-3688.)

One important ground rule is that everybody is a "delegate," and no one is introduced by title or occupation. The sponsors don't want assertive experts making others behave deferentially. The rule works so well that in Williamsburg, one saleswoman delegate did not discover until the end of the day that her opponent in debate, delegate Jerry Baliles, was the state attorney general -- and now governor-elect.

The topics for the day-long debates are chosen from a list of six issues currently in controversy. In addition to the possibility of a new constitutional convention (to write a balanced-budget amendment or make broader changes), the topics include term limits for members of Congress and federal judges, changes in the method of electing the president and his tenure in office, the item-veto and the legislative veto.

Delegates are recruited from churches, clubs, civic associations, business and labor groups. They meet one evening to divide themselves into teams focusing on particular topics. The next morning, those selected by their fellow delegates open the debate on each issue, and then everyone may join in. At the Arlington session, we found that passions were still rising and positions were still being clarified when the 90 minutes for each topic came to a close.

We also found (and Richard Lawrence of the Virginia Jefferson Foundation says this is typical) that the debate quickly came to focus, not on legalisms or mechanistic arguments, but on the fundamental values embedded in the Constitution.

"Do you trust the people or not?" speakers would demand. "Our government was designed to resist the tyranny of the majority," others would reply. What some saw as elitism, others saw as true republicanism. What appeared to some a radical experiment looked to others like a needed tuneup on an overworked engine.

With mounting excitement, we discovered what we had forgotten: that debating the Constitution quickly leads you to think hard about the nature of man in society, of justice, freedom and law. Whether one finds himself thinking, as I do on most issues, that we cannot improve on the original, or asserting, as others did so well, that reverence for the real Constitution requires revision and adaptation of its provisions, the Jefferson Meeting turns into a voyage of discovery to the roots of this republic.

It is a trip thousands of Americans of all ages should make in the next few years. I know of no better way to celebrate -- and revitalize -- our magnificent two-century-old experiment in self-government.