WHEN MONTGOMERY County considered building an indoor tennis center less than a decade ago, officials were worried that there wasn't enough interest to make it a profitable venture. They let a private entrepreneur build it on public land instead. Today the facility is booming and the entrepreneur is making the money.

Arlington officials underestimated the popularity of tennis when they planned the modern Thomas Jefferson Community Center, which opened in 1972. Today, with hindsight, county officials bemoan the fact that their pride and joy is deficient in tennis courts.

More recently, the soccer boom has left recreation and parks officials besieged with requests they cannot fulfill for playing fields.

Such occurences are the rule, not the exception, in the public segment of the $160 billion leisure activity industry.

"We're not keeping up with the change in facility demand," said Bill Hughes, director of Arlington County's department of environmental affairs, which supervises parks/recreation. "We always see ourselves catching up instead of leading."

Indeed, the bicycle boom is about five years old and many people are cycling for recreation and transportation. The Maryland-National Capital Parks and Planning Commission recently drafted a proposed master plan for Montgomery County parks, recreation and open spaces.

The plan called for a 132-mile bikeway system by 1985. As of 1976, only 13 miles of the system had been built; 43 miles were programmed for construction by 1981. Thus, by the time the complete system is functional, biking could be an activity of the past, like hula hoops.

The recreation industry is somewhat like the clothing industry - faddish and trendy.

Leisure-time planners who get 100 per cent enrollment in all their scheduled activities are not considered successful administrators. Failure in some recreational programs is considered a sign of a wsie, creative planner - it usually indicates an innovative approach to developing programs.

If 60 per cent of offered classes and activities are filled, the National Recreation and Park Association figures an overall program can be considered successful.

Some area jurisdictions, such as Prince George's County, schedule one-day workshops in new class ideas in an effort to gauge public interest.

The Washingotn Post's recent six-week survey of the area's public recreation industry showed the following profile of what peopole want:

Community parks with activities for the entire family. Examples: preschool crafts; swimming and organized sports for school-age children: classes and sports for adults.

With mobility curtailed by the energy crunch, the trend is away from big regional parks. Local parks and centers are attractive to young adults (aged 20-40) particularly if babysitting and day-care services are available.

Washington area recreation buffs are big on fitness and classes designed toward creativity. Slimnastics and aerobatic dancing have heavy enrollments; other classes with high registration include sewing, quilting, ceramics, rug-making silk screening.

The traditional concept of public recreation departments - child and sports-oriented - has been replaced in major metropolitan areas by agencies that serve all segments of the population and offer environmental studies, culture and the arts as well as sports.

U.S. News and World Report's economic unit places leisure-time activity as the nation's No. 1 industry at $160 billion a year. In a decade it has surpassed such established industries as automobiles, defense and home building.

The relative newness and rapid ascent of the American "leisure ethic" puts recreation workers in the same position teachers were in 30-40 years ago, before certification, advanced degrees and specialization were standard.

Darrell Winslow, executive director of the Northern Virginia Regional Park Authority, recalled in jest how he became a recreation major at North Carolina State in 1956: "The only reason I got into it was that I thought it would be badminton every day."

Then he grew serious: "There were only seven colleges and universities with parks and recreation curricula then. When I was in college they told us it would be a big industry and engineers at the school said that was wishful thingking."

Today more than 300 colleges and universities offer curricula in parks and recreation. In the past recreation and parks department administrators were either political appointees or middle-aged civil servants waiting out their days to retirement. Now these administrators are professionally trained in parks and recreation.

Only one area jurisdiction, the District, names a recreation director as a political appointment, according to the National Recreation and Park Association.

And the sophistication is reaching such a level that soon, according to industry sources, top administrators will be trained not only in recreation, but in business management as well. Graduates of the prestigious Wharton School and Harvard Business School will be vying for the jobs.

Indeed, it is easy to see why when one visits Fairfax County's ultramodern Wakefield Community Center, a $6.5 million project on 26 acres.

The newer recreation complexes are designed to be self-supporting through fees, taking much of the burden off the taxpayers. Wakefield, according to Joe Downs, director of the Fairfax County park Authority, should generate enough annual revenue ($400,000) to offset its operating expenses.

Montgomery County collected almost $2.25 million in fees last year.

At the Wakefield complex, it costs $1.50 daily to use the swimming pool, gymnasium, handball courts, saunas or weight rooms.The country board, concerned its policies will lead to charges it provides recreation only for well-to-do people budgeted $36,000 to be used for admission passes for lower-and moderate-income residents.

But tax money will only go so far in providing services. Arlington has historically been service-oriented, but the parks and recreation budget has not increased in two years. Community center hours have been reduced and programs cut back.

Senior citizens now going on field trips are required to pay bus fare. In sports, the county used to provide equipment for games. Now the participants provide the gear.

In Montgomery County, recreation director Neill Ofsthun reports a decrease in enrollments as fees increase. In order to keep some fees reasonable the level of service has been decreased.

Many recreation and parks director say they should aggressively lobby for more money from their jurisdictions. Recreation budgets are usually the first ones out.

"That's the case nowadays," and Judd Hockman, director of the Vienna (Va.) Recreation Department. "I do know why. It's an intangible. The people can see a hole in the street or a missing policeman, but they can't see 100 missing softball teams.

"It's always going to be that way. It's a fact of life that I've come to accept over the years."