Four out of 10 commuters from Virginia now take the subway or a bus into Washington during morning rush hour instead of driving to work, Metro reported yesterday.

The use of Metro's subway and bus lines by morning Virginia commuters represnets a 33 percent increase since July 1977 when Metro opened the subway Blue Line to Virginia. Before then, three of every ten early commuters rode buses.

Metro also reported yesterday that overall daytime rail and bus ridership by Virginia commuters had risen by 75 percent since the Blue Line opening.

The rising use of the rail and bus lines from Virginia reflects an increasing overall public acceptance of Metro, reinforced yesterday by new figures showing that every day last month - for the first time in the system's history - an average of more than 250,000 people rode the subway.

The sharp increase in commuter use of Metro subway or buses available was disclosed by a recent count of people who crossed over and under the Potomac River during the busiest hour on a weekday morning.

The survey showed that, of 48,600 people who entered Washington from Virginia between 8 and 9 a.m., 19,800 rode Metro vehicles. Of that number, 13,300 rode trains while 6,500 rode buses.

Automobiles traveling across all the bridges that feed traffic into the city - Chain, Key, Roosevelt, Memorial and 14 Street - carried a total of 28,800 people, a large but dwindling share of the total.

Metro personnel counted the transit riders, while the Metropolitan Washington Council of Governments counted automobile users as part of a survey it has done for many years.

The Metro count showed that the new total of transit users from Virginia from 6 a.m. to 7 p.m. is 57,500, compared with 32,500 prior to 1977. Automobiles brought 166,800 people into the city during the same 13-hour period.

An overall increase in ridership on the rail system also was reported yesterday.

The new figures show that an average of 250,171 fare-paying passengers rode the trains each weekday in May. It was the first time since Metro began initial operations in March 1976 that daily patronage topped a quarter million for an entire month.

The number of riders each day toward the end of May was about 10,000 greater than at the beginning of the month.

"Maybe it's tourists, maybe it's energy, maybe it's seasonal, maybe it's a trend," general manager Richard S. Page told the Metro board yesterday, "but it's encouraging."

Delmer Ison; Metro secretary-treasurer, said rail patronage for May was 38 percent greater than in the same month of 1978, when an average of 180,852 people a day rode the trains. However, the figures are not strictly comparabe, since the Orange Line extension to New Carrollton, in Prince George's County, did not open until last November.

The increase in Virginia transit riding was no surprise to several Metro passengers interviewed as they waited to ride home last night on the Blue Line.

"I've noticed a big difference," said Gayle Morin of Arlington, who works at the Internal Revenue Service. "I stopped riding several weeks, and when I took it again the other day, I was surprised at the number of people crowded in."

Robert Obermeyer of Alexandria said he was one of the new passengers attracted to transit by the rail service.

An employe of the American Telephone & Telegraph Co., Obermeyer said his working hours are irregular and he could not rely upon infrequent bus schedules to get home at night. The frequency of trains encouraged him to park at the Pentagon City station, he said, where he could easily switched to the subway.

Figures from the American Public Transit Association, distributed at yesterday's Metro board meeting, showed that Washington's transit system is experiencing larger gains in riding than other big-city systems.

Washington's patronage was up 18.3 percent in March, as compared with the same month of last year. These gains were reported by other cities: Baltimore, 6.8 percent; Los Angeles, 6.2 percent; New York City, 1.5 percent; Philadelphia, 5 percent, and Detroit, 15.1 percent. CAPTION: Picture, Escalators at the Dupont Circle Metro Station are more crowded these days with ridership on the subway system averaging 250,000 per day. By Vanessa R. Barnes - The Washington Post