Metro, sometimes criticized for inadequately warning riders about major changes, has spent $4,650 to run a newspaper advertisement that invites testimony on how much to raise fares and cut service.

Hearings began last night and will continue next week on proposed fare increases and service cuts to take effect in January, just six months after the most recent fare increase. Metro has identified a possible $12 million shortage in its operating budget if the changes are not made.

The advertisement, a first for Metro, has run in regular display space in The Washington Post, The Washington Star and in some suburban newspapers and has supplemented the small-print legal notices that Metro is required to run.

Great emphasis is placed on the cost of a labor-contract provision that requires Metro four times a year to increase the salaries of its 5,000 bus drivers, train operators, station attendants and mechanics by the same percentage that the cost-of-living index increases. Metro officials say the major factors in the budget problem are unexpectedly high labor costs and dramatic increases in the cost of diesel fuel for buses.

"Clearly," the ad says, "a continuation of this (cost-of-living) formula during a high inflation period cannot be maintained without service and fare adjustments." The only other way to balance the budget would be for local governments to increase their subsidies to Metro, and most of them have said they are not unwilling to do so.

Charles Boswell, president of Local 689 of the Amalgamated Transit Union, said, "I'm not going to let the ad upset me." However, he said, "I don't think it's particularly responsible of 'em. If they want to have hearings on fare increases, I don't think arguments for or against them should be placed in the ad, just presented at the hearing."

An arbitrator recently concluded hearings on cost-of-living protection and other unresolved contract issues between Metro and Local 689, and is expected to issue a final ruling in November. The union has been without a contract since April 30, but Metro is obliged to continue cost-of-living payments and other contract provisions while binding arbitration continues.

About 85 persons attended the first hearing in Falls Church last night. Of the first 20 who testified, about half opposed proposed bus route cuts and about half opposed fare increases. Several opposed both.

Reston commuters were particularly well represented. They could be hit by as much as a 50-cent fare increase for a one-way bus trip to Washington and lose all weekend Metro service.

"This proposal is intended as a technical fix to a political problem," said Ross Hardter of Reston. He said Fairfax County should increase its subsidy of Metro.

Sharon Smith of Falls Church said the proposed elimination of bus route 26 would leave her with "no way to get to my part-time job in Tysons Corner or to church." She owns no car, she said.

Others opposed the elimination of nonstop bus service from the Springfield area to Southwest Washington across the 14th Street Bridge. Metro has proposed terminating many of those buses at the Pentagon subway station.

Metro's newspaper advertisement says, "The challenge of financing Metro's current operations requires hard choices: how high should the fare be, how much local government aid should be appropriated, how much service should be provided?"

The ad says that since the opening of the subway in 1976 "the Metrorail system has been the yardstick by which other transit systems are measured," and says that Metro management has implemented a number of efficiencies to control costs.

Metro General Manager Richard S. Page said that an advertisement was chosen "partly to call attention to our hearings and partly to set forth three or four key financial issues that are the reason for the hearings."