The vacant field of grass that once surrounded the Pentagon City Metro station has sprounted a bumper crop of cars and communters as city bound suburbanites have discovered the subway stop built for the future.

Eventually, the 116-acre site that is now home to a variety of trees and insect species will be covered by office buildings, apartments and a shopping center. For now, with the blessings of Arlington County, it has become a free parking lot for commmuters who find it a way to save time, money and the gritted teeth of rush hour.

Nearly 400 cars a day are parked in the dusty field surrounding the station, and Metro spokesmen estimate that more than a 1,000 persons aday are using the station as a way to get to work.

"We're delighted," said H.S. hulme. Arlington County's transportation director. "The more people who ride the system, the better off we are. A lot of them are people who would normally be driving to work."

Those using the station, many of whom had never heard of Pentagon City before and stumbled on it by accident, share Hulme's satisfaction. "It's saving us time, money and dents in the car," said Walt Jarrett, who sat contemplating the sun in his Karmann Ghia while he waited for his wife, Dianna, to emerge from the underground.

The jarretts live in Lorton, and until Metro opened this month, he would drive his wife into Washington each morning to her job as a travel agent before heading off for his own job in Arlington as a teacher in the county's adult education program. "I used to fill up the gas tank twice a week." Jarret said, "Now its back down to once a week."

Diehard bus commuters have also found Pentagon City to be a money-saver. "I used to say I would never ride Metro because I hate to be under ground," said Margaret Oliver, who works for the Passport Office and lives in Alexandria. That was before she discevered that the subway would cost slightly more than half of what she was paying daily on the bus. "When I found out it was cheaper," she said, "I decided it was metrorail all the way."

Such idyllic tales may not outlast the summer, howere. The county is concerned that autumn rains and winter snowfalls may turn the wide open spaces into muddy and dangous chaos, and plans are now afoot to pave the area over and charge commuters for the privilege of parking there.

According to a spokesman for the Cafritz Company, the owners of the tract. Arlington has asked the company if it would be willing to transform about two acres of the site into a parking lot until it was time for that section of the land to be developed.

Current zoning on the land prohibits such a parking lot, but, according to Hulme, his department will seek an amendment to the ordinance that would permit the company to build the lot.

Such possibilities, howere, do not concern the fortunate few for whom the Pentagon City stop is not a way station in their commute but a ten minute walk from the apartment buildings and homes down the road.

"It's fantastic," said Lynn parker who works for the Food Research and Action Center near the Foggy Bottom-George Washington University stop. "It's cooler, more comfortable, and the people are friendlier. It used to be that I had to turn my emotions off on the way to work. Now I can enjoy myself all the way to the office."