Two For The Show grazes in the shade of a tree at Rokeby Farm, Mr. and Mrs. Paul Mellon's 6,000-acre racehorse breeding and training layout near Upperville in Fauquier County. The mare nibbles, a bit listlessly, on bluegrass and clover, but mainly just waits for an event scheduled to happen at least a week before. Two For The Show is in foal to Kentucky Derby and Preakness winner spectacular Bid, as they say in horse-breeding circles.

"The foal should be on the ground by tour time," predicts Rokeby's stable manager Bernie Garrettson, showing some visitors around the main brood mare barn. Rokeby's brood mare and foal quarters and its yearling filly barn will be among the dozen equine attractions open to visitors at the Virginia Hunt Country Stable Tour to benefit Trinity Episcopal Church in Upperville this Saturday and Sunday.

The 16 clean and spacious stalls in the brood mare barn, trimmed with polished brass, are marked with the names of all the mares that have occupied them and with detailed genealogies of the present residents. By tour time at least some of the stalls will be occupied by mares and nuzzling foals.

"All my mares are in Kentucky now, being bred," Garrettson explains. "We breed mostly in Kentucky. We only breed with a few stallions in Maryland."

Mares, according to Garrettson, a former jockey who grew up around horses in Darlington, Maryland, go into heat shortly after giving birth. To take advantage of this they are shipped off to be bred right away and, since they are all nursing mothers, their foals go with them. Eleven months later, usually in the middle of the night, there will be new foals, delivered by Garrettson, who lives on the farm. By that time, this year's foals will be living in a yearling barn, one of which is also on the stable tour.

"These are last year's babies. We don't name yearlings, but that one is out of Summer Guest by Secretariat," says Garrettson proudly, after pulling his station wagon up to a horshoe-shaped stable decorated with a relief medallion of a racehorse. Several of the 24 grooms employed to care for the approximately 120 horses at Rokeby lead some of the 14 yearlings that live in this barn to a nearby paddock. The horses, perhaps curious about the visitors, linger near the fence for a few minutes before running off. This spring and summer the only thing they have to do is develop the sound bones they'll need for the big business of racing.

"We'll start breaking these horses in September. We don't rush our horses. About August first we'll have riders come in, but I don't care if they don't get on the horses until September. First you usually put on a saddle, but not very tight, and lead them around. You could write a whole book on breaking yearlings," says Garrettson. "We don't train them; we educate them to go where we want them to go -- and quietly."

Training for these horses will begin in earnest in November, at another Mellon farm in Aiken, South Carolina. Then, in April of their two-year-old year, the horses will go to barns at the racetrack in Elmont, New York, site of the triple-crown Belmont Stakes. There, for the first time, they'll gallop -- not race -- around a real track.

"At first we don't even breeze them -- that means let them go fast," explains Garrettson, as patient with reporters as he is with horses. "We don't race them until they're almost three-year-olds, or older, some of them. Not until they're ready."

The yearlings in the paddock have found a tree, shelter from the afternoon sun. As the station wagon goes by, Garrettson seems content to watch them at leisure, growing fit for the years ahead.

"It's a tough grind to be a racehorse," he says. "It takes a tough athlete." SEEING THE STABLES

In addition to Rokeby Farm, stable tour ticket holders will see Tennessee walking horses going through their paces at Mr. and Mrs. Jack Kent Cooke'sFallingbrook; training of horses for international three-day- event riding at Mr. and Mrs James C. Wofford's Fox Covert Farm; the new stables at Foxcroft School; steeplechase horses at Mr. and Mrs. Randy Waterman's Trappe Hill Farm; and several others. Visitors who arrive early on Saturday or Sunday may watch prospective racehorses go through their paces from 8 to 10 at the Middleburg Training Track, where a light breakfast will be available in the track kitchen. Right next door, at the Equine Swim Center, horses will be exercising in the swimming pool. The training track and the swim center are on Route 611, off Route 50 between Middleburg and Upperville. Tickets, $10 for adults and $5 for children under 12, will be sold at all of the above places and at Trinity Church, on U.S. 50 in Upperville. Lunch is available at the church for $5 per person, and during lunch visitors may watch a demonstration of carriage driving by the Piedmont Driving Club.

Another event coming up in Upperville is the 129th Upperville Colt and Horse Show, June 8 through 12 at the horse show grounds on Route 50. There are picnic areas and lunch will be available. On Sunday June 18, riders and horses will compete in the $15,000 Upperville Jumper Classic at 2 p.m., preceded by the $5,000 Rotating Pair Relay.