The Arlington County Board took the first step last night toward ending the program that reduces Metrorail fares for people who take short trips within the county, even though by almost any measure, the program has been a success.

Subway ridership in Arlington is up, commuters who travel only one or two stations are happy, and hundreds of Pentagon and Crystal City workers now use the bargain 60-cent fare -- a quarter cheaper than Metro's basic fare -- to have lunch or shop at the Fashion Centre, the mall that opened last fall near the Pentagon City station.

The problem is an increasingly tight revenue picture. The county is reconsidering whether it can afford the reduced-fare program, which officials expect to cost Arlington about $150,000 in fiscal 1991 and much more as subway ridership increases.

The County Board's action last night was to ask Metro to hold a public hearing on County Manager Anton S. Gardner's recommendation to consider dropping the reduced-fare program, which is financed by Arlington and the state.

"If we had a lot of money, it might be very nice indeed to continue this . . . but we really cannot afford to," said County Board member Mary Margaret Whipple, who is chairman of Metro's board of directors.

Arlington officials said the move against the reduced-fare program, a tiny piece of Arlington's $412 million budget for next year, reflects the lengths to which the county is going to try to cut costs.

"This isn't to say {the reduced-fare program} doesn't have its positive aspects, but you have to weigh it against other needs," Gardner said. "We have a lot of programs that could use $150,000."

Arlington's financial outlook was much brighter in December 1988, when county officials seeking to increase Metro ridership reduced subway fares for trips of less than two miles within the county as part of a six-month experiment. Metro's basic subway fare then was 80 cents, and the reduced Arlington fare was 50 cents. The discount program's fare was raised last summer, when Metro's basic fare was increased to 85 cents.

"When we began this, the whole economic picture of the Washington area was different," Gardner said of the discount program. "Now, we're looking at this program and others and questioning whether they are necessary."

Arlington's Transportation Commission, an advisory panel, has urged the County Board to keep subsidizing fares for short subway trips.

In a letter to Board Chairman Albert C. Eisenberg, commission Chairman Charles O. Bishop Jr. warned that ending the discount fares could prompt commuters to seek other means of transportation and could lead to "increased roadway congestion, air pollution and waste of productive time."

Metro riders interviewed yesterday echoed Bishop, and several said they have used Metrorail more often because of the discount fares.

"I'd hate to see them go back to the full fares," said Rosslyn resident Benjamin McCall. "I got into a habit of taking Metro more because it's cheap."

Gardner's move to end the reduced subway fares was brought on by projections that increases in state aid to the program won't keep pace with its costs, leaving Arlington to fund an increasingly larger share.

The program will cost Arlington about $120,000 this year, Gardner said. It is projected to cost $150,000 next year, and would rise steadily if ridership increases continued.

County staff members said the reduced-fare program in some ways has become a victim of its own success, and that combined with the rising activity at the Pentagon City Metro station it is threatening to become a larger financial burden than the county is willing to bear.

Edson L. Tennyson, planning coordinator for the county's Department of Public Works, said that before the Fashion Centre opened last fall, about 3,000 people used the Pentagon City station each day. About 9,000 people now pass through the station daily, many of them on the subsidized fares.

"I think you'd have to call this a successful program," Tennyson said. "It's just a matter of whether we can afford it."