Gov. Harry R. Hughes reversed himself and agreed today to support legislation that would contribute $25 million more for Maryland's share of Metro construction costs, extending state aid for the Washington subway system through 1981.

Hughes, who had previously opposed further state assistance until currently appropriated funds run out next year, had come under increasing pressure in recent weeks from Washington suburban legislators who have demanded an immediate commitment from the state.

Gene Oishi, the governor's press secretary, said Hughes still believes the $40 million already set aside for Metro construction will cover expenses through the end of next year when the money appropriated in 1972 is expected to run out.

But Hughes decided to change his course, the press aide said, to "accommodate" Prince George's and Montgomery county legislators who feel their county governments need assurance of additional state aid when preparing next year's budgets.

Although Oishi insisted that suburban legislators promised nothing in return for Hughes latest offer, an administration source explained that the governor feels that he needs the help of the two large Washington area delegations to pass his administration's legislative tax package.

"There were no specific deals cut," said the aide. "But obviously these people have been putting up a lot of pressure. If Harry's gone so far to please them, obviously he expects some good will when they're considering his programs."

Hughes revealed his decision in a meeting with three Washington area senators -- Peter Bozick, the chairman of the Prince George's Senate caucus, Victor Crawford, the chairman of the Montgomery County senators, and Laurence Leitan, chairman of the senate Budget and Taxation Committee, which handles Metro legislation.

The three senators have been pushing legislation that would require the state to pick up their counties' share of future Metro construction costs -- $56 million in Maryland -- to complete the 100-mile transit system over the next five years.

In the privacy of his office, Hughes told the senators he was willing to support their bill if it was limited to $25 million, enough to carry the two county governments costs at least one more year after the funds run out.

The governor also agreed to set up a task force after the legislative session to explore ways to finance Metro construction for the next four years and to pay for the annual operating deficits that must be covered by the counties.

"A half a loaf is better than none, especially when you're starving," Crawford said after the meeting. 78It will give us a little breathing time so our administrations can count on the funds for next year's budget."

The compromise announced by Hughes today carries political risks for the first-year governor. A bill that would have mandated $156 million more in state aid for Metro construction was killed in a Senate committee last week. To put the Hughes proposal into effect, this measure would have to be revived and amended to reduce the aid to $25 million.

If the Washington suburbanites can get the amended measure out of committee -- they say their chances are good with Hughes' support -- they would most likely face organized opposition on the floor from a coalition of Republican, conservative and rural senators.

That same bloc postponed approval of the Baltimore subway with an eight-day filibuster in 1976, frustrating the Baltimore senators and their allies from the Washington suburbs.

Senate Minority Leader Edward Mason, a leader of the 1976 antisubway talkathon, began marshaling the opposition forces today moments after learning of Hughes' plan. Mason promised that he would mount another filibuster at least as strong as the one three years ago.

"I think he's [Hughes] lost his mind," said Mason, a Western Maryland Republican. "We should come out of here with a program that gives tax relief to taxpayers, not state aid to wealthy subdivision [Prince George's and Montgomery]."