When the River Place cooperative opened for sales in Arlington's Rosslyn section several years ago, newspaper and television advertisements portrayed an army of young, upwardly mobile professionals marching across Memorial Bridge from the District into Arlington County.

And, true to the ad campaign, the yuppies have descended en masse upon Arlington -- not just in Rosslyn but throughout the county's close-in Metro corridor -- snatching up condos, town houses and, whenever possible, single-family homes.

The message was simple, and one that real estate agents say they have known for years: Arlington is a mecca for young adults, particularly if they are singles or childless couples.

"Arlington is really the hottest spot in the metropolitan market" for young adults, said Don Shamblin, vice president of Better Homes Realty.

About 38,000 persons between the ages of 25 and 34 moved into Arlington in the decade between 1970 and 1980, accounting for a 33 percent increase in their numbers since the 1970 census. That makes the age group the fastest-growing and single largest in the county, representing 25 percent of the county's 153,000 residents.

While county officials welcomed the increase in the young and upwardly mobile, they have become concerned that the same statistics that told of an increase in young adults also showed what some call an alarming drop in the number of families choosing to remain in Arlington.

After years of study, the county recently issued a brochure in a campaign that was the antithesis of the River Place message. A color map shows a cartoon mom and dad with three children following the rainbow to Arlington, with highlights of the county's services and facilities available for families.

"Clearly Arlington is attractive to lots of young singles and professional married couples who don't have children," said Mary Margaret Whipple, vice chairman of the County Board.

"They are a fine addition to our community. But what also needs encouragement is for families to come into Arlington and stay," she said recently.

While it may be too soon to tell whether the campaign for families is working, it's clear that the movement of large numbers of young, single adults into the county has not abated.

Phil Barodin, 38, a political analyst with the Justice Department, said he moved to Arlington from Bethesda almost eight years ago. "There's not a lot of riffraff here. It's basically a yuppie environment," he said. "I ride home on the bus with women in $200 suits and $50 tennis shoes."

Alice Lippert, a 32-year-old economist with the Energy Department, said she likes the county's international flavor, the relatively low taxes and the services. "I like the older homes. Doesn't it sound yuppyish to say you like older homes?"

Just as Alexandria has Old Town and the District has Georgetown and Capitol Hill, Shamblin said, Arlington's trendy spots are the Crystal City and Rosslyn-Ballston Metro corridors that have become "very fashionable hot spots in the market."

"Every house I've sold in Arlington, with the exception of one house, has been sold to young people," said Jane Davison, who has worked for Town and Country Realty for two years. "They're professionals and they want to be close to the city."

For residents such as Steve Courtney, a 28-year-old resident of the Fairlington town house condominiums in South Arlington, the social and recreational aspects are important as well. "I like the class of friends you meet here," Courtney said as he stopped by one of the complex's 12 pools after a Saturday morning basketball game with friends. "Everybody has a good job, everybody makes big bucks. Socially, you're right there."

The impact of the group can be seen in what is now being offered by the county's adult education and recreation programs.

The Arlington adult education program, for instance, has added courses that appeal particularly to young adults. New Orleans cookery, interior designing, basic vegetarian meals and microwave cooking classes fill up as fast as most of those the recreation department offers in aerobics and tennis.

Connie McAdam, chief of the county's Recreation Division, said there has been a significant increase in the number of young adults signing up for programs. More recently, she said, she has been asked to allow a Washington area Frisbee club space for its national "ultimate Frisbee" tournament -- a football-style sport played with a Frisbee, which is popular among recent college graduates.

Cwi Steiman, the managing partner for the Courts Royal fitness centers in the area, says the Arlington club has a younger membership than others in the region. "We've always catered to a population that is young in Arlington," he said.

Some of the young adults who have come to Arlington in the last decade have stayed and started families. Charles Whitestone, a 39-year-old mortgage banker, moved to Arlington from Alexandria seven years ago with his wife Ellen. A freak lightning storm that damaged the upstairs apartment of their condominium town house in Fairlington eventually led them to knock out walls and redesign the house for what has become a four-member family.

"We wanted to stay here," said Ellen Whitestone. "We're happy with the tax base and the proximity to wherever we want to go. The county has excellent child care and preschool programs. It fits the bill for what we wanted."

But she worries that, years from now, the family may have to move to another county in search of a larger house.

It's the kind of predicament that has county officials worried.

"We want to maintain a balanced community which has all age groups," Whipple said. "We need to stress what the advantages are for families here. We don't want to become a single-adults community by default."

Between 1970 and 1980, the average size of a household in the county dropped from 2.43 persons to 2.07. The number of families decreased 21 percent while the number of one- and two-person households increased 20 percent. By 1980, single persons of all ages accounted for 55 percent of the county's population.

But there are signs that families are coming to stay in Arlington, Whipple said, referring to indications of a "minibaby boom" in the county and elsewhere.

While Arlington had the lowest "fertility rate" in the area in the last decade, more residents have begun having babies, state figures show. In 1982, women aged 20 to 34 years had 1,881 babies, compared with 1,599 in 1980.

The increase in children also can be seen in the enrollment jumps in day-care facilities and the preschool and afterschool "extended-day" programs conducted at all elementary schools. Both programs have waiting lists at some facilities.

County school enrollments, after a decade of steady decline, are beginning to stabilize. In the 1984-85 school year, said C. Thomas Weber, director of operations and facilities in the schools, enrollment increased to 14,684 from 14,525. Enrollment projections for the fall are for 14,895 pupils.

The county's School Board, concerned that only 19 percent of Arlington's households have children under age 18, has decided this year to emphasize the importance of the school system to the residents who do not have children but who pay taxes to operate the schools.

Young, single adults and childless couples realize the importance of the school system, if only because it affects property values, said Libby Ross, a Town and Country real estate agent who was part of the county's task force on families.

"They used to move out, but we are having families moving in now," she said. Ross said she has confidence that the number of families will grow in Arlington: "A yuppie, really, is a professional couple that's decided to have babies."