Rivalries between area governments broke into the open again yesterday as Metro board members debated how to vote when a final decision is made next Thursday on a proposed fare increase.

Board members appeared to have reached a consensus to raise base rail fares from 65 to 75 cents. But unity broke down over mileage fees that decide many riders' total rush-hour rail fares.

Representatives from D.C. and Virginia, whose constituents tend to take short rides on Metrorail, favored raising the mileage fee from its current 13 cents to 15 cents per mile. Relatively few of their riders would be affected by such an increase, because mileage fees apply only after three miles of travel.

Board members from Maryland argued for a compromise of 14 cents. Residents of the state tend to take longer rides, commuting from New Carrollton and Silver Spring, and are harder hit by mileage increases.

Yesterday's debate came during a meeting of the board's budget committee, which had convened to make a recommendation to the full board. Motions to recommend a 75-cent base fare with a 15-cent mileage fee and a 75-cent fare with 14 cents mileage were both defeated.

Finally, Maryland voted with D.C. board alternate Hilda Mason, who has long advocated free fares, and passed a resolution calling for rail fares to remain at the current 65 and 13 cents.

Board Chairman Richard Castaldi of Prince George's County afterward played down the recommendation. Maryland voted with Mason as a "posturing" meant to create a stronger bargaining position when the full board makes a final decision on the fares next Thursday, he said. The committee made no recommendation on bus fares.

The committee also defeated a motion by Maryland alternate Carlton Sickles to recommend that the coming increase be in force for at least 15 months. A fiscal 1984 budget that the board is now considering assumes another fare rise after 12 months.

The fare package the board is considering also contains a 5-cent charge for bus transfers. D.C. board member Thomas Downs said the city agreed to accept this in exchange for a 15-cent mileage fee. If resistance continues to the 15-cent fee, he said, "we would have to seriously reconsider this position."

D.C. has traditionally opposed charging for transfers, because relatively large numbers of its riders use them. Maryland, where transfers are fewer, has supported the charge on the grounds that it would increase revenue and reduce transfer abuse.

The new fares would be implemented on April 2. As proposed by Metro staff, rail base fares would rise to 75 cents, with the 15-cent mileage fee. Bus base fares would also go up to 75 cents, except in the District in off-peak hours, when they would be 70 cents.

Some bus zone fees would rise, as well as special fares for the elderly and handicapped.