ALONG THE fence lines of the vast empty meadow, birds are singing. The black and white dog stands quietly at its owner's side as a horseman drives five sheep onto the far end of the field. The dog tenses and the man leans to murmur in his ear. Then, like a bullet, the dog races in an arc hundreds of yards wide to get behind the sheep. For a long moment, hidden by the sheep, the dog disappears from view. The horseman canters away and, suddenly, the sheep raise their heads and start toward the dog's owner. For the next 12 minutes the man whistles continually -- trills, urgent sharp notes, gentle reassurances -- and the dog swerves left or right, stops dead in his tracks or hurries forward. "Dot!" the man calls, and Dot drops to her belly as the sheep gather around the man's feet and, leaping for joy, escape back up the course. Not for long. Dot comes around to haunt them through a pair of free-standing white gates, two hundred yards away.

This dog executes split-second commands while hundreds of yards from its owner. When Rachel, my 12-year-old niece, watched her first sheepdog trial, she set her hands on her hips and remarked, skeptically, "These are dogs, right?"

From Saturday through Sept. 26, historic Belle Grove Plantation in Middletown, Va., will host the National Finals Sheepdog Trials, the biggest, most important trial in North America. From Canada, Texas, Wyoming and California, 150 dog athletes will compete for the national championship.

Sheepdog trialing is a graceful, gentle sport the whole family can enjoy. The dogs execute their intricate maneuvers to whistles not much louder than fence line bird calls. The sheep behave so well spectators sometimes ask if they have been trained to act their part in the drama.

These trial sheep are from a flock that earns its keep grazing underneath power lines in New Hampshire during the summer months and travel to Florida to gobble down kudzu all winter. The sheep are an environmentally friendly replacement for herbicides, an "ecology flock." And no, they're not trained.

One hundred and twenty years ago, the sheepdog trial was designed by men who made their living from sheep; consequently these sheep are handled very gently. Should any dog nip a sheep, it is promptly disqualified and its owner (who may have traveled from California for this moment) gets zero for a score.

Some of these dogs work farms or western ranches, and for them this trial is a busman's holiday. Other dogs sleep on suburban couches and visit farms on weekends to work sheep.

The human competitors are ranchers, veterinarians, librarians, CPAs. Half of the top 20 handlers at last year's finals were women.

In the 18th century, Belle Grove was owned by the Hite family -- friends of Thomas Jefferson, who helped design the beautiful manor house. Jefferson was America's first sheepdog importer and it is likely some of Jefferson's sheepdogs once worked this great meadow.

Nursery dogs (under three years of age) will compete Saturday and Sunday, from 9 a.m. to dusk.

The Open dogs will run vie in qualifying runs Wednesday, Thursday and Friday to winnow 150 to the 50 that will compete in the semifinals on Sept. 25. The top 20 from the semifinals will compete Sept. 26 to determine the champion North American sheepdog.

Some say the sheepdog trial is the most difficult test of human/dog communication ever devised. Trial organizer Wink Mason, who brought the finals to Virginia, is more modest, "It is very difficult. Your dog is working a long distance away and the relationship you've built up where you trust your dog and he trusts you . . . that relationship is constantly tested."

During the championship runs, each dog will be asked to run out nearly half a mile to gather his sheep. Then, as he fetches them back to his owner, he will be asked to abandon these sheep and run out a second time for a second batch of sheep. He combines the flocks, fetches them to his owner, drives them away through two sets of free-standing gates and back into a 100-foot circle where it first becomes apparent that five of the sheep are wearing ribbons around their necks. In that small ring, the dog and man must extract the five ribboned sheep -- and no others -- and put them into a free-standing, 9-by-12-foot pen. When the pen gate is closed, the dog's work is over.

Families who enjoy nature shows as well as dog and animal lovers should appreciate this intense, gentle, sometimes hypnotic sport. After the first few runs, everyone understands what's going on and most pick favorites. Ten-year-old children will probably get as much out of the trial as their parents; younger children may get restless after a half-dozen runs.

When young children are tired of watching, you might want to take them on the Belle Grove house tour (included in the admission price) for a change of scenery. Also, on Sept. 24 at noon, experts will demonstrate explosive-detecting dogs and on Sept. 25 at noon there will be a demonstration of narcotics-sniffing dogs. There are food and craft vendors in the big tent behind the mansion or you can bring your own picnic.

Though most of these dogs are friendly, children should be warned that some are not, and the owner's permission must be sought before petting any dog.

Children can visit and throw Frisbees for the dogs at the Border Collie Rescue Booth. These once-abandoned dogs are retrained, people-oriented and (parental warning) looking for homes.

For those who've never seen a sheepdog trial, the nurseries, qualifying or semifinals will be just as satisfying as the championship and a good deal less crowded. There are benches for spectators, but you might wish to bring your own folding chairs.

Mason says of the event, "Often after people watch these sheepdogs working they'll come up to me and say, `These dogs are better behaved than my kids.' "

"I just smile."

NATIONAL FINALS SHEEPDOG TRIALS -- Belle Grove Plantation, Middletown, Va. Take I-66 to Exit 302, go west on Route 627 to U.S. 11 in Middletown, south one mile on U.S. 11 to Belle Grove. 540/869-2028. Web site: Nursery trials are from 9 a.m. to dusk Saturday and Sunday. Qualifying rounds are 9 a.m. to dusk, Wednesday, Thursday and Sept. 24. Semifinals are Sept. 25 from 9 a.m. to about 5. Championships are Sept. 26 from 7:30 to 5. $6, 10 and under free. Parking is $1.