The Reagan administration wants to slow the pace of Washington's Metro subway construction and is studying possible cuts in the planned 101-mile system, according to informed sources.

The plan is part of the belt-tightening President Reagan has promised for almost all domestic programs. But it also relfects a recurrence of the anti-rail system attitude held strongly by some conservatives in the Carter administration.

According to the sources, no specific decisions have been made about unfinished lines of the Metro system, but it has been decided that Metro will have to compete equally with other rail programs in the United States for federal funds.

Furthermore, there will be a shortage of those funds because, all administration sources agree, expensive rail construction is a prime target for budget cutting for the forseeable future.

The administration is known to oppose special appropriations for Metro as were authorized in the $1.7 billion Stark-Harris bill that overwhelmingly passed Congress last year. Congress, of course, may decide to disagree with Reagan.

Metro is assured of 61 miles of subway, administration sources agree. Thirty-seven miles are in operation and 24 more are fully funded. Partial funding has been acquired for about 20 more miles. The final 20 miles are neither partially funded nor under construction.

One administration source said that although there is "no specific proposal to cut Metro back to 60 miles, obviously, everything is under review." The source added that "general recommendations" for cutting spending in the area of transportation "might well have implications for Metro."

Those last 20 miles will be regarded as "new starts" by the administration, and thus will be treated the same as hoped-for-rail systems in Los Angeles, Detroit and Houston, according to the sources.

It is unclear what status the 20-mile, partially funded but uncompleted segment will have, another source said. "All of that is under discussion right now," one source said.

The uncompleted line that would be most seriously affected by a cutback is the Green Line, which will run through Washington's inner city from Greenbelt on the north to Rosecroft Raceway on the south.

The Congressional Budget Office, a longtime opponent of new rail transit systems in general and Metro in particular, also weighed into the battle this week by suggesting that the federal government could save money by abandoning the Metro program at the endof 1982 with 69 miles fully funded.

Metro officials have expected for some time that their construction schedule will have to be extended beyond 1990, the current projected completion date.

The Carter administration had promised Metro $350 million in federal funds for 1982 -- the same amount Metro has already received this year -- plus a supplemental appropriation this year of $60 million.

The sources said there is no chance for the supplemental and that the 1982 amount will definitley be cut. It could conceivably be eliminated.

Transportation Secretary Drew Lewis was reportedly told by advisers recently that if he murdered his wife on the Mall, he would get less unfavorable press than if he tried to cut the Metro system.

Rep. Michael Barnes (D-Md.) said yesterday that "it would be a serious mistake to cut back on public transportation, particularly on a project that has received incredible support from the Congress . . . I certainly hope this is only a rumor."

Metro General Manager Richard S. Page said that "Metro has already gone through two careful reviews of its system -- once in the late 1960s and again in the late 1970s."

"Both times it has been confirmed as cost effective," he said.

Page sent hand-delivered letters yesterday to Lewis and Arthur Teele, Urban Mass Transit administrator-designate, asking for early meetings to discuss the Metro program.