CITY SMART: How to thrive in the urban environment of Southern California. : Off-Road, After Hours : Welcome to the Fast-Growing Sport of Night Mountain Biking. Only One Problem--It’s Illegal.

In pitch darkness, paramedics hiked into Orange County’s Trabuco Canyon last month to rescue an injured mountain biker.

As they hauled him out on a stretcher, bobbing lights flashed around a bend in the trail and several bikes whizzed by, narrowly missing the emergency crew.

It was the paramedics’ introduction to a fast-growing and still-illegal sport in California: night mountain biking.

Nobody knows yet how many Southern California cyclists lug their expensive off-road bicycles over government fences posted with “No Trespassing” signs to hit the wilderness trails after dusk.

But state and county rangers have gotten reports of nocturnal bike tours of up to 40 riders per night.

Southern California bike shops and manufacturers also report booming sales of battery-operated light systems that cost as much as $500.

Some professional riders have caught on to the trend to take advantage of the cool mountain air during training sessions for mountain bike races.

Other, less experienced bikers cruise the dark mountain paths for light exercise in pairs--looking like wobbly automobiles in the night as they hash out relationships or discuss money market accounts.

And with darkness setting in about 5 p.m., mountain creatures are finding they must share their world more frequently with two-wheeled intruders during this time of year.

“Most people can’t ride until they get off of work,” said Terry Martin of Fontana. For three years, Martin has been racing on an eight-mile track in the foothills above Claremont with about 30 regular nighttime riders.

Below a full moon Wednesday night, the cyclists zoomed at breakneck speed underneath a canopy of drooping branches.

“It’s a surreal experience,” Martin said. “Much better than daytime riding. It’s almost like a drug. Once you start, you can’t stop.”

State and county officials are less enthusiastic about the pastime.

For safety and environmental reasons, they are trying to get riders to quit their late-night jaunts into the wilderness.

In Orange County, where the trend is now at full throttle, state rangers regularly hand out misdemeanor citations to cyclists roaming the trails after sundown.

County officials who have learned about the dangers of night riding from environmental groups and bicycle safety advocates have begun issuing warnings to park trespassers as well.

“We don’t allow night activity--period,” said Orange County parks manager Tim Miller. Night cyclists “are people breaking the rules. They could ruin it for everyone else.”

Park officials in the Los Angeles County area, however, do not yet see the new sport as dangerous enough to merit full enforcement of its nighttime ordinance.

“I haven’t heard that it’s a problem,” said Bertha Ruiz, the park trails coordinator for the county. “We really don’t have the manpower to enforce against something like that. If it gets to be a problem, we’ll focus on it more.”

But with nobody minding the parks at night, officials from both areas are worried about liability issues and accident response.

“Accidents are our main concern,” said Orange County’s Whiting Ranch Ranger John Gannaway, who stopped nine mountain cyclists during a special night patrol last month.

If riders get badly hurt, nobody will be around to aid them, he said.

Just as important, Miller added, is the disturbance cyclists cause for coyotes, mountain lions, raccoons and other nocturnal foragers in the Southern California wilderness.

“These animals are searching for food [at night],” he said. “People riding through there with their lights, noise and speed upset the wilderness habitat.”

Although the cyclists pose a hardship for wild creatures, riding at night eases the crunch that results during daylight hours when cyclists, hikers and equestrians compete for limited trail space.

“You don’t need to worry about running into other people at night,” said Jeffrey Neal, who rides regularly after dark in the Pacific Palisades and Topanga Canyon. “The only drag is riding in the water, because it gets really cold.”

The growing popularity of night riding has not been lost on entrepreneurs.

Several manufacturers make high-priced light equipment of all sorts for mountain biking.

“The market is expanding, “ said David Matsu, store manager of Performance Bikes in West Los Angeles.

And Tobin George, whose Pasadena-based company, Nightsun, manufactures one of the more popular lighting systems, said his company’s sales have doubled every 18 months since 1987.

NiteRider Technical Lighting of San Diego has sold more than 30,000 helmet and handlebar lights for mountain bikes in three years. Company spokesman Mike McGary says sales have increased by about 50% in each of the last three years.

George said the more expensive lighting systems available have dual beams ranging in power from 10 to 45 watts with rechargeable batteries that will last for about two hours.

The stronger beams, he said, look much like motorcycle headlights, capable of beaming a cone of light up to 100 feet ahead of a rider.

“It’s no worse than driving a car at night,” said Dave Mashaw, president of an Orange County mountain biking club called Share.

But even if those lighting systems illuminate the mountain trail adequately, there are safety issues to keep in mind, said Dan Gutierrez, member of a statewide group called the Concerned Offroad Bicyclists Assn.

“You want to ride a trail you’re already familiar with,” he said.

Still, Gutierrez, who describes a night ride in Mexico where an owl flew silently by his right shoulder for about a third of a mile, suggests that more people should try the emerging sport.

“It’s definitely a rush,” he said. “You’re part of the nighttime scene.”