The cost of driving through the Baltimore Harbor Tunnel and over that city's Francis Scott Key Bridge increased 25 cents for passenger cars yesterday, the first day of rate changes at Maryland's six toll facilities.

The changes will help pay for the new Fort McHenry Tunnel on I-95 and for road and bridge repairs, the Maryland Transportation Authority said. Toll rates in Maryland had not changed since 1975.

The rate for passenger cars at the tunnel and the bridge increased from 75 cents to $1. Trucks will pay 12 to 17 cents more for each axle. Passenger cars will pay $1 at the Fort McHenry Tunnel when it opens later this year. add d Man Guilty in Pot Case Brian J. Murphy, 30, of 1466 Greenmont Ct., Reston, pleaded guilty in Fairfax Circuit Court yesterday to a single count of manufacturing marijuana after state police found pot plants growing in his town house.

Murphy faces a minimum sentence of five years and a maximum of 30. Judge F. Bruce Bach set sentencing for Aug. 23, and Murphy remained free on $20,000 bond. A second charge of possessing marijuana with intent to distribute was thrown out by Bach.

According to yesterday's hearing record and a search warrant on file, state police seized 138 pounds of marijuana during a search of Murphy's residence Jan. 17, as well as about a dozen marijuana plants growing in a closet and 65 seedlings sprouting in a bedroom.