The Senate adopted legislation yesterday that includes a 26 percent cut in federal spending for Metro subway construction this year, casting renewed doubt on the transit authority's plan for expanding the rail system to 89.5 miles by the early 1990s.

The allotment for Metro, contained in an overall $10 billion bill to finance the nation's transportation programs, was reduced to $184.5 million, $65.5 million less than the transit agency had sought and the lowest level since the mid-1970s.

The House previously recommended a $237.5 million appropriation for Metro, a 5 percent cut described as acceptable to transit offficials. Metro's advocates have said they will urge a Senate-House conference committee to endorse the House-backed allotment.

Sen. Charles McC. Mathias Jr. (R-Md.) warned in a statement accompanying the Senate legislation that the proposed cut in funds for Metro would "undermine this carefully crafted agreement" to complete 89.5 miles.

The 89.5-mile plan provides for a Green Line extension connecting Fort Totten in Northeast Washington with a Greenbelt terminus in Prince George's County, and a Yellow Line spur to a Van Dorn Street station in Alexandria's West End.

The Senate Appropriations Committee previously recommended a 25 percent cut in Metro funds to $187.5 million. This amount was reduced by an additional $3 million yesterday as a result of a 1.6 percent across-the-board cut in transportation spending.

Among other major reductions included in the Senate measure were a $300 million cut in funds for the Federal Aviation Administration and a $200 million cutback in Coast Guard outlays. Some senators said they will seek to restore these funds in the Senate-House conference.