When 12-year-old Peter McGowan won the senior division of Washington's 1982 Metropolitan Soap Box Derby recently, he beat his 14-year-old sister Martha in the two-person final race.

It wasn't the first time the McGowan family of Rockville has dominated the derby. Last year, Peter won the junior division championship for youngsters aged 10 to 12. In 1979, his older brother, Paul, won in the senior division for 12- to 15-year-olds.

Their father, Paul Sr., who heads an architectural firm in Washington, looks on these triumphs with no little satisfaction. "I've been involved in Soap Box Derbies since 1950 when I built cars for my brothers," he said. "But this hasn't happened to my family before."

The McGowans plan to attend the All-American Soap Box Derby in Akron, Ohio, Saturday to watch Peter compete in the national races. It will be the fifth year they have gone, thanks to earlier wins by Peter, a student at Robert Frost Junior High, and Paul Jr., now a student at Thomas Wootton High, and other triumphs by a neighbor, Hugh Flood, who drove a car that McGowan designed, and a cousin.

The McGowans' soap box reign could go on for years. Martha, who will attend Thomas Wootton this year, is eligible for next year's championship, and 7-year-old Tom McGowan will be old enough to race in three years.

This year's races, sponsored by the Greater Washington Soap Box Derby Association and the Downtown Jaycees, attracted 30 drivers who competed for two spots in the All-American Derby.

Under the derby's rules, when a driver lost, he or she was placed in the losers' bracket, where another loss resulted in elimination. The winner of the losers' bracket raced the undefeated contestant for the championship.

As it turned out this year, Martha lost in her first race of the day but went on to became the top person in the losers' bracket and was matched against her undefeated brother in the two-person final. The McGowans raced each other twice in the final. Martha won the first time, but lost the second, leaving Peter the ultimate champion.

At the start of both finals, Peter and Martha hummed inside their cars, sounding like a swarm of bees. "It transfers vibrations through the car, which may make it quicker," said Peter.

The light-hearted family rivalry and the friendly spirits of the drivers made the hot, muggy weather seem less oppressive. Several hundered spectators erected canopies, ate ice cream and drank soda and beer while the drivers battled for four hours.

"I couldn't believe I won this year," said Peter, whose final winning time was 30.22. "It was like one of those game shows when you answer the question right and win the prize. I thought I beat Martha in the first race because I couldn't see her car. When I found out I lost, I got really serious."

Martha, who also competes in another form of racing called soap box rallies, said afterward, "I wasn't disappointed losing at Washington because I've been pretty successful at the rallies." But, noting how chatty she had been before the finals, she added, "My dad tells me I talk a lot when I'm nervous. I guess he's right. I was trying to break the tension."

The drivers build their own cars out of wood or fiberglass under strict specification and with limited guidance from their parents. Junior division cars must be 80 inches long, 13 inches high and 14 inches wide and cannot weigh, with driver, more then 220 pounds. Senior division cars are 84 inches long and either 13 inches high and 13 inches wide or 12 inches high and 14 inches wide. The car is limited, with driver, to 250 pounds. Lead weights are added to bring cars up to the maximum weight.

Margaret McGowan, mother of the champions, said, "I'm getting a little tired of it because it takes up a lot of time. There are a few things around the house I'd like to do. But it teaches the family to work together. They relate real well.

"I remember at Christmas I asked if we could stop talking about derbies and talk about Christmas," she said, laughing.