The Office of Management and Budget, where subway systems have long been regarded as unsavory budget-busters, yesterday agreed that money raised by the gasoline tax increase can be spent for new rail transit systems.

OMB spokesman Edwin L. Dale Jr. announced the agreement as if it were a new administration policy. In fact, it was a public acknowledgement that the Reagan administration has decided it is committed to spending at least some of the new gasoline-tax revenue on so-called "new starts."

That doesn't mean that the administration is embracing new subway lines warmly, but it does say that Houston and Los Angeles--both of which have ambitious subway plans--are major political constituencies.

The story started last year during Senate Finance Committee hearings on the proposed nickel-a-gallon gasoline-tax increase for highway and transit improvements.

Sen. Lloyd Bentsen (D-Tex.) said he had serious questions about the bill if there would be no money for new starts, which, in Bentsen's case, meant Houston's planned 18.5-mile, $2.1 billion rail system, which is supposed to include two miles of subway.

Drew Lewis, then secretary of transportation, announced at a subsequent committee session that he had discussed the matter with the OMB and that some of the $1.1 billion in new annual transit aid could be used for new starts. Bentsen said he would vote for the bill.

But as an OMB official said this week, "There has been a dispute here about whether Drew Lewis was properly committing the administration." Dispute or not, Dale said yesterday that the OMB would not oppose "new starts" as long as they are "cost-effective."

Houston still isn't home free. The $150 million in federal aid it is seeking for fiscal 1984 must come from discretionary transit grant funds for which many cities are competing.

Yesterday the head of the federal Urban Mass Transportation Administration recommended that money for new starts be deferred for yet another year so that the backlog of rail modernization projects can be reduced.

However, when the issue was raised, UMTA Administrator Arthur E. Teele Jr. said that proposed subways for both Houston and Los Angeles rank very high on UMTA's list of potentially cost-effective transit systems.

Houston's system has substantial local backing and is supported by a 1-cent local sales tax.

Bentsen said late yesterday that the administration has "made a commitment to me. They ought to live up to it, and I think they ultimately will." CAPTION: Picture 1, David A. Stockman . . . agency views subways as budget-busters. Picture 2, Drew Lewis . . . had promised $1.1 billion for new rail systems.