"Riders, take your places," the voice of Sal Zeuner booms over the loudspeaker.

At his command a pack of teen-aged boys scrambles to the starting gate for the first bicycle motocross race of the afternoon.

The thrill of competition fills the air.

Perched atop their bicycles, which each entrant has assembled himself, the riders survey the course. Carefully they adjust their helmets and pull down their face protectors.

They crane their necks over the padded handlebars, place one foot on the forward pedal and the other firmly on the ground. For a split-second the riders appear frozen.

"Go!" shouts the official.

The starting gate drops and tearing down the dirt embankment comes a swarm of T-shirted cyclists.

At the first curve they disappear into a cloud of dust. After three successive bumps two riders are down. The pack continues, pedalling their hearts out through the snake-like course.

This is bicycle motocross racing in Rockville - relatively safe, clean (if you discount the dirt and dust) fun for teenagers.

Sixty-one racers between the ages of 5 and 17 participated in Sunday's inaugural race on the newly completed bicycle motocross course at Gude Drive and Taft Court next to the Red Gate municipal golf course.

Construction of the course was a joint effort between the Department of Parks and Recreation and area bicycle motocross enthusiasts.

The city donated the land to build the 800-foot course, but it was the teenagers who dug it out and found companies to donate the straw bales and tires needed to provide safe shoulders for the track, said Ron Olson, head of the city recreation department.

Motocross bicycles have a 20-inch wheel base and are equipped with heavy-duty wheels and reinforced handlebars.

The object of the sport is to "get the right geometry for racing and the lightest weight bike," said Zeuner, of the National Bicycle League. He acted as the race official at the event that attracted youths from Maryland, Virginia and New Jersey.

Because the race was sanctioned by the national organization, winners will be eligible to race in national competition.

Motocross racing began about six years ago in Florida and California. In Rockville, it became so popular that city streets overflowed with riders. Some residents didn't approve, and when police began chasing the teens off their makeshift race courses, a parents' group known as the Rockville BMX Association was organized to help teenagers find a site for a permanent race course.

Jeannie Lackey, whose sons Brian, 14, and Robbie, 13, are racers, asked the Rockville City Council for help. She said it was "totally supportive" and within days made land available. The course will be open for racing daily at designated hours and on Sundays. The teens using the course have volunteered to maintain it, Olson said.

At races, parents act as judges and inspect bikes to insure they meet racing criteria.

"The main criteria is that they're safe," said Jack Cole, father of racers Peter Cole, 17, and Gary Cole, 13. Riders must wear helmets and long-sleeved shirts, and have parental permission to ride.

Ruth and Newton Moore are almost as enthusiastic about the sport as their son Paul, 13. "We think it's a terrific activity," Ruth Moore said, adding that her son does yard work for neighbors to earn the money needed to buy parts for his $175 bike.

While the sport in this area is dominated by males there were two female riders from New Jersey in Sunday's race. Jennifer Zeuner said she thinks more girls don't ride "because they're afraid they will fall and get hurt." The worst injury she sustained has been a scratch on the hand, she said.

For the Crickenberger family of Glen Echo, motocross racing is a family affair.

Brothers Fred 11, Chucky, 9, and Brian, 5, all participate. Their father Fred comes along to cheer. "I think this is great," he said, helping Brian off his bike after the first race of the day.

Sunday's winners were: In the 8-year-old and under division, (1) Keith Hutchins of Pasadena, (2) Mike Griffin of Kensington, (3) Paul Plummer of Silver Spring; In the 9 and 10-year-old division, (1) Daniel Martin of Columbia, (2) Jennifer Zeuner of Vineland, N.J., (3) Mike Walen of Rockville; In the 11 and 12-year-old division, (1) Robbie Mylis of Reston, Va. (2) Gene Webb of Columbia, (3) Chuck Brown of Clarksville.

In the 13 and 14-year-old, (1) John Bezerungen of Columbia, (2) Steve Sordo of Kensington, (3) Bill Hermach of Rockville; In the 15-year-old and older division, (1) Tom Moran of Columbia, (2) Matt Bezerungen of Columbia, (3) Scott Brown of Clarksville.