Across Key Bridge from Georgetown sits Rosslyn, the upstart Manhattan skyline at the edge of the green trees and two-car garages of the Virginia suburbs.

On Friday Morning, the Metro subway will come to Rosslyn, adding the last, long awaited major component of this born-again section of Arlington County.

Eventually, say the entrepreneurs with a stake in Rosslyn's future, Metro will be the key that unlocks its potential. It will bring, they say, the life that is lacking now, the consumers, pleasure seekers and tourists that now see Rosslyn as the means to the other end of Key Bridge.

For the present, say Arlington County planners and developers, Metro will mean chaos. In a little over two weeks after Metro opens, about 80 buses will come to the end of the line in front of Rosslyn's brand new $42 million subway station. About 40,000 commuters will then be riding the second longest subway escalator in the Western Hemisphere.

There is no terminal for the buses. There is no sidewalk for the commuters. There is only a pile of dirt, a vacant lot, and N. Moore Street, a four-lane road providing a challenge to pedestrians that may come to resemble the crossing of the Beltway on foot at rush hour.

By day, when 20,000 people are working there. Rosslyn hums to the tune of corporate and governmental giants. They do business in buildings ranging in style from Neo-Bastille to Fantasy-in-Concrete. The line-up, as listed on the first floor office directories, runs the gamut of the federal directory and the Fortune 500. Xerox, State Department. Defense Intelligence Agency. IBM. CIA. Honeywell. Even EST, the California consciousness-raising program, has moved its local offices there.

By night, however, because only 4,000 people live there, Rosslyn resembles a long abandoned movie set. Yesterday's newspapers scuttle across deserted, dimly lit streets, their path unbroken by sidewalks yet to be built. No trees catch the sign of summer breezes, and the telephone poles are decked with signs pointing to restaurants and hotels that are nearly impossible to reach because of highway construction.

At Arlington County Board meetings, where planners, board members and residents recently spent long hours developing a general land use plan for the county, Rosslyn, with its lack of parks and life after dark, became at times the code word for what not to do in the future.

In the early 1960s, when the building began, Rosslyn was the future. Until then, Rosslyn had been the wrong side of the tracks in Arlington. It was a seedy collection of palm readers and pawn shops, lumber yards and storage tanks. The population seemed to consist, for the most part, of the derelicts asleep in the drainage ditches and the occasional carnival or fundamentalist religious group that came to town.

Now the high rollers work where the holy rollers prayed and the only church in town sits above a gas station. Looking down on it all, at the hour of the day when cares melt away with the ice cubes, a group of co-workers conducted an informal seminar of sorts on Rosslyn from the vantage point of Alexander's Three penthouse restaurant.

"Rosslyn is Concrete City," said Rich O'Brien, a young salesman with Lexitron Corp., which is headquartered in a nearby building. "Up and down, no grass, no trees, no community. It's like a big neuter. It takes people to make a community and people are something this place doesn't have. To me, it's a place where you come, you do business, and you leave."

But to John Howard, a former Lexitron employee now working in Old Town, Alexandria, Rosslyn captures all the galamor of the bussiness world, all the glitter that comes with success and all the gritted teeth it takes to achieve it.

"This is it, this is where the action is," Howard said. "There's more business going on in Rosslyn than in any part of Washington. It's convenient - when you get anywhere from here faster than you can from Washington. And it's got class. There's nothing phony here, it's all up front."

On Sunday evenings, Rosslyn lies quiet in the fading summer sun. The great glass windows mirror gold from the sunset. The occasional car driving up Lynn Street toward Key Bridge provides the only sign of motion.

There are those who live there who wouldn't have it any other way.

Three women sit on a balcony in London House, one of two luxury apartment buildings built during Rosslyn's reconstruction, their conversation interrupted periodically by the jets screaming toward National Airport.

Through their eyes, Rosslyn becomes a small town, where the hair dresser in the next door office building is the conduit of all the gossip, the hotel restaurants the local hangouts, and the apartment swimming pool the place where friendships are made.

For them, the lack of people in Rosslyn has reduced the areas towering architecture to manageable proportions.

"It's cool that there aren't very many people here," said Linda Lee Gardner, a young secretary who works for a trade association in Washington. "I come back from a party and see the lights on my friends' apartments and I feel like I'm coming home."

"The filling station man knows us, the people in the drugstore know us, the people in the grocery store know us," said Marie Cantwell, who works for the National Automobile Dealers Association at Tysons Corner. She came to Rosslyn seven years ago because then her job was there and stayed because now it's home. "There's a real community here, and there aren't too many places you can say that about in Washington."

The parks that are missing in Rosslyn itself are only minutes away, they say, and the area has its own special esthetic. "You never get used to watching the mist over the Potomac," said Betty Thompson, an Arlington lawyer whose office is in a nearby high-rise. "To look down and see a thunderstorm roll in over the river - it's something I'd never want to leave."

But the very tranquility so pleasing to Rossyln residents tends to make Rosslyn hotel managers and merchants stare mournfully heavenward. Throughout the litany of complaints, however, one constant note of hope is sounded. Metro is coming. And the merchants of Rosslyn talk about Metro the way drought-striken farmers talk about rain.

"There's going to be an explosion here," said Thomas J. Lemon, manager of Hyatt House, Rosslyn's newest hotel. "With Metro, people will finally realize that this bank of the Potomac isn't 50 miles away, that we really are a part of Washington."

Since it opened a year ago, the Hyatt has catered mainly to Rosslyns built-in clientele, the out-of-town businessmen who come to Rosslyn to see clients and heads of corporations.

now, Lemon said, the hotel is planning a new publicity campaign. The hotel will advertised in the subway stations as the one closed to the Rosslyn Metro. In addition, he said, the subway will give the hotel the opportunity to go after the tourist trade that wants "the excitement of Washington and the tranquility of Virginia."

It will be a welcome change for Lemon, whose advertising recently has had to take a different tack. One ad blown up to poster size in his office depicts an aerial view of Rosslyn with the slogan, "Located somewhere in this general vicinity," a humorous reference to the fact that the rerouting of Rosslyn traffic in the last two years makes finding the hotel an adventure in itself.

Lemon echoes the optimism of many of Rosslyn's businessmen when he talks of Metro providing Rosslyn businesses with a kind of captive audience. "The potential is fantastic," he said. "With all those commuters getting off and on their buses here, people are going to have to take a look at Rosslyn."

What the first commuters will be looking at however - at least for the time being - is a rather forlorn scene that won't be changing much for the next three years. Sidewalks and easy access to the surrounding buildings won't be available from the Metro station until a planned office building is completed sometime in 1980.

The confusion arises out of the vagaries of the process under which Rosslyn was developed. In exchange for County Board approval of their building plans, developers in Rosslyn are required to provide their own curb, gutter and sidewalks, as well as street lighting and connecting pedestrian walkways on the second level.

The Metro station will have all this and more. Beginning in August, a 21-story office building will be under construction. Eventually the building will top the metro station with two stories of commercial stores and restaurants and provide commuters with walkways to all the surrounding buildings.

In three years, predicts the developer, Stanley R. Zupnik, the building and the station will be "the hub, the center of action in Rosslyn."

Until then, predicts Arlington County longrange planner John Gessaman, "It's going to be a mess. Everyone's just going to have to grin and bear it."