In a decision that officials say could cost Montgomery County millions of dollars, a federal judge has ruled that most of the county's 714 career firefighters are eligible for overtime pay after 40 hours a week under the federal Fair Labor Standards Act.

The decision, issued Thursday by U.S. District Judge Herbert Murray in Baltimore, also may add incentive to making the firefighters county employes, rather than private employes as they are now, county officials said.

The career firefighters in Montgomery County are employed by 18 private corporations that make up the county's fire and rescue system, using both the career employes and more than 1,000 volunteer firefighters, a setup unique in the Washington area.

The firefighters union sued 15 of the 18 corporations in May 1986, seeking back overtime pay to May 1983. They contended that as employes of private corporations, they are eligible for overtime after 40 hours.

Most firefighters work 48 hours a week, according to union attorney Thomas Woodley.

Under the Fair Labor Standards Act, private employers are required to pay overtime for any hours worked past 40 hours in one week, but employes of government agencies are not eligible until they have worked 53 hours, Woodley said.

The corporations, noting that the firefighters are paid with county funds, argued before Judge Murray that they do not come under the 40-hour requirement because they are "quasi-government" entities and also are not engaged in interstate commerce as required by the act.

Judge Murray brushed the arguments aside, saying that he did not believe "the 'public' nature of the corporations outweighs their 'private' nature." Also, Murray said, the firefighters' work affects interstate commerce because they are asked to put out fires along interstate roads with traffic crossing state lines.

"The cost of this kind of court ruling could be in the millions," said Robert Kendal, director of the county Office of Management and Budget. "Now the County Council has another reason to consider changing the employment status of the career firefighters" to county employes.

The ruling comes one day after a special committee report was released suggesting sweeping changes for the county's fire and rescue system, including making career firefighters county employes.

Such a system has been proposed several times since 1940 and has been opposed and defeated by politically influential volunteers in the county each time. Volunteer firefighters have said they will fight it this time, too.

Any changes recommended by the report would have to be drafted into legislation and voted on by the County Council, which seems to have mixed opinions on the issue.

"This decision makes it almost certain that we'll make {the career firefighters} county employes," said council member Bruce Adams.

Council member Michael L. Subin agreed but said the primary issue is whom the firefighters answer to: the county or the corporations run mostly by volunteers.

But council member William E. Hanna Jr. said he is "going to do everything I can to defeat" the proposal.

He said the change would cost millions of dollars and would kill the county's volunteer system.

"I don't think the volunteers are going to take this laying down," Hanna said.

"What they ought to do is resign tomorrow in a block . . . and let the union firefighters put out the fires," he said.