She was wearing green running shorts and light blue Nikes and as she loped along the Potomac, her legs creamy tan in the springtime dusk, the softball player was struck mid-beer. Love.

"Can I run with you?" he called as she passed. The legs were even better up close, he noted.

"Sure," she called back. She always said sure. It was safe because, first, nobody ever took her up on it, and second, this guy had cleats on.

But oh, the agonies that have been endured for love. Cleats were a small thing. He fell into place beside her, beer in hand, Budweiser softball cap on his head. She thought he was cute.

They ran all the way around the Tidal Basin, clackety-clack. Her name, she said, was Isie Pfeifer, a 23-year-old George Washington University student.Her hair was long and dark, pulled back in a neat ponytail. His name, he offered, was Jim Cassedy, a 25-year-old insurance salesman from Annandale. His hair was blond, his face a little wet. The date was May 14, 1979. Washington smelled moist.

They ran to his car, where he changed shoes, then up toward the city, up toward GW, up toward 22nd and I. Five miles. "Just about killed me," he says now. "I was trying not to let her know."

They stopped at her apartment. Feeling frivolous, she invited him in.

Love on the run, you could call it. As spring blossoms on the streets of Washington, so do the joggers. Thousands and thousands of the, thighs jiggling, hips quivering. They litter the Mall at lunchtime, greet dawn on Memorial Bridge, scamper along the towpath. Most just jog on by each other, sweating and brooding. But others, snared by a bright T-shirt or meaningful glance, grunt hello.

This can flower into a second grunt the next day, followed perhaps by a bashful squint. If things seem to be moving along, one or the other may, after much consideration, actually switch directions in pursuit. The first line: "Well, uh, do you run here often?"

So begins the love affair of the jogger. It could be considered a new social form, springing from the physical self-absorption of the '80s with its roots in the same old bar scene that spans the decades.

Now, searching for The Relationship has moved to the great outdoors. It's not as fattening as beer and not nearly so obvious as "Can I buy you a drink?" Clever openers aren't required; in lieu of a comeback, there's always a pantor a wheeze.

"It's a kind of shared experience that may help some people get over the initial difficulty of meeting each other," says George Washington sociology Prof. Thomas Courtless. "It's an atlernative. And it's different than tennis. You have to already know somebody to play tennis.Jogging is sort of spontaneous."

Then, too, forget clothes. "You don't have to be dressed up and pretend that you're not sweating or that your eyebrows aren't plucked right," reasons Nan Shepard, a Washington lawyer who's been running for years.

And the courtship, generally, can be cheap. "Having someone say, 'You wanna run 10 miles this weekend?" is a much more relaxing way of getting to know someone than sitting in a smoke-filled bar somewhere," says Jeff Darman, a former president of Road Runners of America.

The prevalence of this new social form is hard to gauge, given that estimates of runners in the Washington area range from 3,000 to 300,000 -- and also given that any estimate of galloping love affairs is something nobody will begin to guess at. But it's there.

"When you run with people," says one woman who married the person she ran with, "and suffer that much -- you know, stops for water, and stops for bathroom crises -- I don't know exactly what you learn about somebody, but there's a sort of communal feeling."

And it's there even for the most dedicated runner, who after a while gets bored with the immobile scenery like everyone else. Suddenly, out of the corner of his or her eye, pops a leg. So smooth . . .

For Al Morris, an assistant professor of physical education at the University of Maryland, it was an umbrella that broke the barrier:

"It was raining like mad," he begins, recalling a downpour he found himself standing in while waiting to run in an Alexandria race, "and I went up to this nice-looking lady and I said, 'Can I get under your umbrella?' Even in old running clothes and baggy sweats and everything, I could tell that this was an attractive runner."

"I didn't have a chance to answer yes or no," remembers the attractive runner, whose name is Ann, "but we just immediately started talking." They stopped for the race, and afterwards resumed talking. That was two years ago. sNot they're married.

There are other techniques for meeting a fellow jogger. Darman, now associate publisher of Running Times magazine, suggests that good running manners, as in, "Do you mind if I run along with you?" might be the best approach.

You can run up to a certain person," he explains, "and if that person doesn't run away, your existence has been acknowledged."

Once this has been accomplished, and provided that one is in good shape and therefore able to form words, the next sentence might be something along the lines of: "So. How far are you running today?" Assuming the answer is not a marathon, the conversation can usually flow from here into how long you have been running, races entered, marathons entered, marathons finished, the relative merits of various running shoes . . .

Not exactly scintillating, perhaps. But safe. Besides, a discussion of the current global situation could cause many joggers to jog off into the sunset.

(One jogger, who is now living with another jogger she first noticed in a race, tells of meeting him one night on foreign turf, at a party. She blathered politics, she remembers.

("He was bored," she says. "I think he was real bored. But he was polite.")

("I remember thinking that she talked a lot about politics," he recalls later, and confirms: "It was boring.")

But back to technique. A good one, for a jogger who runs routinely with another jogger whom he or she would like to see become more than just another jogger in his or her life, is this: When stopping at a water fountain, scoop up a handful of water and pour it down your companion's back. Lovely.

"I think that might have been what started it," recalls one young Washington woman of her romance with a lawyer/marathon runner. They met on the C&O Canal towpath.

Which is generally considered the best place to fall in love and run at the same time. The second-best place is the Mall. The worst is I-395.

Bear in mind that these are guidelines, not hard and fast rules. Still, the towpath generally gets the best reviews from regular joggers who trot up and down, day in and day out, along the narrow route. The narrowness, depending on your mind-set, is its best and worst asset: worst, because in so doing it creates conversation. It's also a fairly regular crowd that uses the path. This, too, encourages conversation and, in some cases, outright pursuit.

"I saw this runner out there in the mornings sometimes," says Bobbie Conlan, a writer at Time-Life Books, "and then one morning, he caught up with me and ran all the way home."

This was one of two romantic interests and several on the towpath. Then she met Chuck Evans, a nice young lawyer, in the towpath parking lot last spring. She married him in the fall.

As a runner's playground, the Mall cmes in second because it is bigger, with paths spreading out toward the Lincoln Memorial, the Tidal Basin and Arlington. This tends to curtail running intimacy. The crowd isn't quite as up for it either, being more lunchtime, jog-a-quick-five-miles-then-head-for-the-showers-before-the-senator-comes-back types. Few have time for socializing.

But, oh, is there looking.

"I like to watch the men," says Debra Garrison, 20, an FDA clerk-typist who has worn a T-shirt that says "I May Not Be Perfect, But Parts of Me Are Excellent." The men, she adds, "are sexy."

"I do it to watch the women so we can discuss it later in the showers," adds Richard Kurzberg, 29, a computer programmer who jogs with Garrison on fine spring days.

"Jesus, there she goes again," exclaims a non-jogger standing near the Smithsonian. "I've picked out my favorite," he says, pointing to the favorite and at the same time declining to reveal his name and the reason he's in town, which he brands some top-secret Defense Department training.

"I have fallen in love on a number of occasions out here, but have never pursued it," says Pat McCann a transportation budget analyst in the Congressional Budget Office. Well, why not?

"He's married," somes the response from his jogging colleague, who is Chip Conley, an economist in the same office.

Which brings up another subject, that of illicit love. This, too, occurs on the run. But whether it's as prevalent as the more acceptable variety is really difficult to gauge. Yet it's there. And again, much easier to spark outdoors than in a smoke-filled bar.

"If you get out of the house to run with somebody," explains Darman, "99 percent of the time it's innocent. I know of people who have stumbled upon other people making love in the bushes."

Not at this point, it would be nice to say that Isie Pfeifer and Jim Cassedy, after a sweet summer of running together, fell madly in love.But they didn't. And there's not even a catastrophic falling-out to make for a great ending.

What happened was this: After she invited him into her apartment, they had beer and hot dogs and watched the Bullets game. They went out a lot during the summer, either running or to ball games and stuff.

And then, like myriad others who meet in a bar or while waiting for a bus, they fizzled.

"Just kind of wore off," says Cassedy.

"No big deal," says Pfeifer.

But she's still running. And looking.