On the theory that most citified congressional eggheads need to be shown that breakfast does not come from a box, that wily old country caucus on Capitol Hill staged a science fair yesterday.

It featured such unlikely agricultural exotica as maple-flavored yogurt from Vermont, Edam cheese and fancy wine from Mississippi, alfalfa chocolate chip mint cookies from Wisconsin and fish goodies from Massachusetts. And then, there was Planned Piggyhood from Missouri.

All of that, of course, could come from a box. But it starts out on some farmer's north 40, and that was the point of this first National Agricultural Research Fair--to show legislators where their next meal is coming from.

It also was a rather persuasive demonstration by the House Agriculture Committee that the best way to a legislator's fiscal heart is through his stomach. Copious amounts of farm-produced wines and cheeses were rolled in to help do the job.

In these parlous budgetary times, agricultural research has come upon a tightening of the federal pursestrings. One point of the fair was to remind legislators that such items as alfalfa cookies and maple-flavored yogurt--new ways of using old products--don't occur without the research that goes on in Department of Agriculture and state land-grant university laboratories.

So House Agriculture Chairman E. (Kika) de la Garza (D-Tex.) and Edward R. Madigan (R-Ill.), the ranking GOP member, enlisted state extension services to put two dozen of their most futuristic agricultural research projects on display for the uninformed urban legislator.

Their idea, they said, was to "give you an appreciation as to why support funds are needed for these types of research, how these funds are being used and where these funds are going . . . to see the work our scientists are doing to give American consumers a bountiful, safe and price-reasonable food, fiber and forest-products supply."

City lawmakers have heard that before, but few may have seen it until yesterday.

For example, Louis Wise, vice president of Mississippi State University, and Rodney Foil, director of extension at the university, were brandishing bottles of muscadine grape champagne (yes, from Mississippi) and showing off cheese developed there.

Boris Stojanovic, head of MSU's wine program, claimed that his main duty was as a grape stomper, but then got serious long enough to suggest that oenological studies there hold great hope for providing southern farmers a major new source of income from wine grapes.

Alexander Duthie of the University of Vermont's dairy foods program was puffed up like a proud father as white-coated aides passed out samples of a new and supremely tasty maple-flavored yogurt they've developed. At the same time, they were showing off their "new" milk--a low fat, high protein version of skim milk that is actually palatable.

"We're excited," Duthie said. "With this great dairy surplus we have in our country, we're coming up with new ways to utilize milk. That's what this research does."

For Duthie's milk, Richard Straub of the University of Wisconsin had the perfect accompaniment--cookies laced with protein extracted from alfalfa, an offshoot of research that found money-saving ways for farmers to harvest hay.

"This research has gone on for 10 years at Wisconsin, involving many disciplines," Straub said. "We can take protein from any lush green plant--pea vines, carrot tops. It's already being tried in tortillas in Mexico . . . think of the possibilities for feeding people in underdeveloped areas."

And the cookies weren't bad, either.