Senate and House conferees agreed last night to provide $227 million in federal funds this year for Metro subway construction, an appropriation described as adequate to carry out the transit authority's long-debated plan to expand the rail system to 89.5 miles by the early 1990s.

The allotment, approved at a closed session, was announced by Reps. Steny H. Hoyer (D-Md.) and Frank R. Wolf (R-Va.), who have pressed for sizable federal outlays for Metro. A spokesman for Hoyer hailed the agreement as "a great victory." A Wolf aide termed the funds "adequate" to meet Metro's goal.

The transit authority had sought a $250 million allocation this year, a spending level initially endorsed by the Reagan administration. The House recommended cutting the allotment to $237.5 million, and the Senate later voted to reduce the appropriation to $184.5 million.

Metro officials had warned that the Senate move threatened to disrupt the authority's plans for building sections of the long-delayed Green Line in Southeast and Northwest Washington and in Prince George's County. The Green Line is Metro's only unopened route.

Last night, a spokeswoman for Metro General Manager Carmen E. Turner said Turner was "extremely pleased with the appropriation level" approved by the conferees. "I think we can manage with that level for fiscal 1986," said Metro spokeswoman Beverly R. Silverberg.

Despite the Senate-House agreement, Metro's construction plans face considerable uncertainties. The Metro appropriation was included in an omnibus spending measure, which the White House has said it may veto.

In addition, Metro officials and congressional aides have warned that the transit authority's allotment may be jeopardized by a landmark balanced-budget bill also under consideration by Congress. The legislation is expected to mandate further cuts in domestic programs, such as transportation spending.

Moreover, the Reagan administration has recently held up funds previously allocated by Congress for Metro construction in an attempt to put pressure on the transit agency to negotiate a contract to regulate spending. Metro officials say this move may delay work on a Red Line extension to Wheaton.

Even if Metro receives sufficient federal aid to complete 89.5 miles by the early 1990s, the authority still must contend with an estimated $2 billion shortage of funds to reach its goal of constructing a 103-mile system. The additional money is needed for major sections of the Green Line and other routes.

Much of the $227 million allotment is earmarked for two Green Line sections. One of these would extend from Anacostia to a U Street station near 13th Street NW, scheduled to open in 1990. The other, expected to open in 1992, would run from Greenbelt in Prince George's County to Fort Totten in Northeast Washington.

Although Metro and congressional officials expressed relief at avoiding a steeper cut, the $227 million appropriation represents the lowest allocation for Metro in eight years.

A Wolf spokesman said the conferees also approved funds to widen Rte. 1 in Arlington and Alexandria.