"RIDERS READY?" The starter's command cuts through the hazy Sunday sunshine. "Pedals ready? GO!" The low wooden barricade drops flat, and three six-year-old Novice riders wobble away from the starting line, furiously pedaling pint- size bicycles around the dusty race track that snakes to a finish some 800 feet away.

Half an hour later, the 17-year-old Expert racers flash across the same hilly course, intensity in motion, each alert to capitalize on a competitor's error, the split-second slip of man or machine that separates a first-place finish from second or third.

This is bicycle motocross -- BMX -- and the bikes are those blocky little jobs with 20- inch wheels, spread-eagle handlebars and imagination-tingling names (Diamond Back Super Viper, SE Quadangle and P.K. Ripper, to cite a few) that comprise 40 percent of all bicycles sold today.

BMX racing is the fastest-growing kids' sport in the United States, according to the National Bicycle League. Last summer nearly 200 riders tested their pedal power for the first time at the Rockville BMX track alone.

Seventeen-year-old Chris Boudreaux of Lanham, an Expert racer with a reputation around Washington area tracks as "a heavy-duty rider," has found the chief attraction of racing changing during his four years of berm warfare: "First it was the trophies and then the competition, and now it's a social thing. It's a lot of fun to get out there with the guys. Everybody has a good time."

Adds his mother, Penny, "It's sort of addictive. You go to a race or two, you win a trophy, and you . . . are . . . sunk."

The sprocket's siren song snags parents as well as children. "It's a family sport," Penny Boudreaux explains. "The families are all out there."

Classes for competition are divided into age groups ranging from "Five and Under" to "Seventeen and Over," all subdivided into Novice and Expert categories. There is a Beginner designation for those who want to give the sport a trial run. Adventurous adults can ride in the "35 and Over" Cruiser (24-inch wheels) class.

Mainly, though, BMX is for kids and was in fact invented by kids. It began in Southern California in 1969, when a group of youngsters persuaded a park attendant to let them ride their bikes on dirt trails meant for motorcycles. A teenage motorcycle racer subsequently organized the activity, using his own trophies as prizes. Within two weeks, he had 150 riders eager to participate.

The sport rapidly attained international dimensions, and some of those early competitors, now in their 20s, pull down $50,000 annually as professional riders.

BMX offers the flexibility missing from the team-oriented sports generally available to children. An individual can race as often -- or as infrequently -- as he or she desires. Racing can be done for the sheer fun of it, or the rider can adhere to a serious training program (sprint riding, riding up hills, practice starts, calisthenics).

"You're in control of how much you do," Boudreaux says, adding, "The more you do it, the better you are at it."

BMX is also financially flexible. After the initial fixed expenses are covered, the outlay can be relatively minor -- or staggeringly major.

Safety considerations make a helmet with mouthguard mandatory ($40-45). Clothing requirements can be met just as well by a long- sleeved sweatshirt, jeans, cotton gloves and cheap sneakers as by brand-name paraphernalia marketed for style-conscious competitors ($125 and up for a complete outfit).

The BMX racing machine can be anything from a home-built thrasher to the Hutch Pro Star, manufactured by Baltimore-based Hutch BMX Products to sell for $799. (Two fine bikes for young racers are the Mongoose California, under $190, and the GT Mach One, under $250.)

The bicycles are also subject to safety regulations: Handlebars, stem and top frame tube should be covered with BMX pads, and the chain guard, reflectors and kickstand must be removed. The bike must have a number plate ($7 up), though a paper plate with inked-in number will suffice.

Local tracks operate under the sanction of either the National Bicycle League (NBL) or the American Bicycle Association (ABA), and a rider must be a member of the appropriate organization to compete. The NBL has an annual membership fee of $20; the ABA's dues are $25, with discounts for additional family members. Both organizations offer short-term beginner memberships ($5). Benefits include insurance and a monthly newspaper listing the points each racer garners in competition.

The new member's first race is free. Subsequent entry fees are $7, with higher rates for special events such as double points races, which often give three-foot-high first-place trophies. Some tracks feature beginners' nights ($5 entry fee).

Race days and times vary, but all tracks have a 90-minute registration period starting two hours before the races begin. When registration is complete, moto sheets are posted to tell each rider when to compete. ("Moto" means a group of riders of the same age and general ability who will race one another.)

The groups of riders are then summoned to the "staging area" behind the starting gate. Each moto is sent on its way in turn until every group has raced three times. The NBL tracks award trophies on the basis of combined performance in the three races. The ABA system gives every rider three chances to qualify for the final (trophy) race held for each age/ability category.

A day at the races finds friendship and advice dispensed in generous amounts, since veteran riders, parents and track officials are all eager to help newcomers do their best. Occasional spills provide breath-catching moments, but most injuries are no worse than those incurred in street riding, and the young racer rises to finish his turn around the course.

The sport can get you in one way, however. Many a youngster (and his parents) find the weekend activity turning into a full-time passion.

Or, as an 11-year-old devotee puts it: "I don't know what we'd be doing if they hadn't thought up BMX."

GETTING STARTED

Neophyte racers can join up at any BMX track (Maryland ABA tracks are currently offering free membership to NBL cardholders) or request additional information from the NBL, 84 Park Avenue, Flemington, NJ 08822, 201/788-3800; or the ABA, P.O. Box 718, Chandler, AZ 85224, 602/961-1903.

Youngsters wishing to sample the sport before committing themselves are welcome to ride the tracks before and after races, as long as they comply with all safety requirements.

FACTS ON TRACKS

For the child who wants to compete or just watch, racing continues at area tracks until November and resumes in April. (Howard County BMX has an indoor winter season.) Call for specific race times and directions. Here are the tracks and some coming events, a number of them this weekend.

MARYLAND

ROCKVILLE BMX -- An NBL track, Seven Locks Road at West Richie Parkway, Rockville, 890-7466 or 460-4607. Racing this Sunday (at noon), Sept. 28, Oct. 12 and 20.

COLUMBIA BMX -- (ABA), Harpers Farm Road (behind Joseph Square Village Center), Columbia, 301/261-0674 or 301/636-4578. Saturday (at 1), Sept. 28, Oct. 12, 19 and 26 and Nov. 2.

HOWARD COUNTY BMX -- (ABA), Howard County Fairgrounds, Route 144, West Friendship, 301/489-7095 or 301/854-6561. Sunday (at 1), Sept. 29, Oct. 6, 13, 20 and 27.

SOUTHERN MARYLAND BMX -- (NBL), Post Office Road (behind St. Charles Bowl), Waldorf, 301/743-3709 or 301/645-4924. Saturday (at 4 p.m.), Oct. 27.

BMX -- (NBL), Route 75 at Route 80, Monrovia, Md., 301/831-6921 or 301/831-8141. Sept. 29, Oct. 4, 5 (Maryland State Championship), 6 and 19.

MILLERSVILLE BMX -- (NBL), Newcut Road at Gambrills Road, Millersville, Md., 301/544-2403. Oct. 13 and 26.

VIRGINIA

LAKE FAIRFAX PARK BMX -- (NBL), Lake Fairfax Park, Route 606, Reston, 971-7959. Sunday (at 1:30), Sept. 28, Oct. 6, 13, 20 and 26, Nov. 3, 9 and 10 (Virginia State Championship).

Sally Palmer is a Washington writer and the mother of BMX racers.