The Upperville Colt and Horse Show is based upon tradition. Not much has changed here at the Grafton Farm show grounds since the first show in 1853, and the creaky wooden grandstand is evidence that this is, indeed, America's oldest horse show.

One class that brings together all the customs of the show ring and the hunt field is the ladies side-saddle hunter hack class. The event goes back to the days when it was considered taboo for a woman to show up in the hunt field astride a horse.

But today, Katylou Brittle and her mount, Lots of Spots, broke the tradition of the event when they took home the first-place trophy.

The written rules for this competition require certain accouterments such as a silk hat and veil, gloves and hunt whip. The unwritten rules, especially in Virginia, say a nice, quiet horse of a dark color is preferred.

"I thought it was possible that he (Lots of Spots) might not win because of his color," said Kathy Doyle of Middleburg, owner of the spotted horse. "But he carried himself so well and he's a real ham, and I knew he could outjump all the others."

Lots of Spots did outjump the other dozen entries who performed in the main ring. Clearly half the women had some difficulty negotiating the two fences they were asked to jump. Brittle, 27, of The Plains, Va., took both jumps with ease, despite the thick mud. It was the first time she had competed on the 8-year-old gelding.

Brittle is a riding instructor at Foxcroft School in Middleburg, Va., and has been riding sidesaddle for 18 months.Although most of the riders in the show this week will talk about how hard it is just to ride with one leg on each side of the horse, Brittle passes off this potentially dangerous diversion as easy.