The Metro board gave final approval yesteday to the Dec. 5 opening of the Red Line subway extension form Dupont Circle north along Connecticut Avenue to Yuma Street NW, but only after assuring Montgomery County that service would not deteriorate at the Silver Spring end of the line.

The new subway service will bring higher fares and will also mean sharp cuts in Connecticut Avenue bus service.

Early opening of the Red Line extension, six months ahead of schedule, had raised fears that current service on the subway system might be disrupted because of a shortage of rail cars, now being manufatured in Italy. At an earlier meeting, the board had postponed a final vote on the matter until yesterday so that Maryland representatives on the board could consult with officials in Montgomery and Prince George's counties.

The transit authority eased some of Maryland's concerns yesterday by agreeing to add six more subway cars to the 250 now in use on what will become a 39-mile rail system. The new line will add three stations to the existing system, at Woodley Park-Zoo, Van Ness-UDC and Cleveland Park.

The board left unsettled for the moment Montgomery County's concern for commuters who now rely on express bus service to downtown along Connecticut Avenue. That direct service is scheduled to be ended when the rail extension opens, and bus riders will have to transfer to the subway at the Van Ness-UDC station to go downtown.

"The travel time will increase and so will the fare," Art Spengler, deputy staff director for the Montgomery County Council, said yesterday. He said the change would cost some former bus commuters $3 a day for a round trip, instead of the $2.40 they pay now, and would add eight minutes to the 10 minutes it now takes to go by bus from Van Ness to the downtown area.

Metro official estimated that opening the line early will produce a net saving of more than $400,000 because of the bus service cutbacks and the higher rail fares. Similar reductions in bus service were made in Virginia when the Pentagon and Rosslyn subway stations opened.

The Montgomery County Council outlined its reservations in a letter to Metro General Manager Richard S. Page and plans to raise the issue of commuter discomfort at future hearings. Metro officials still could decide to keep some express bus service along the Connecticut Avenue route.

Despite the questions raised yesterday, Maryland's representatives on the board joined in the unanimous approval of advancing the opening of the Red Line extension. Metro officials estimated the early opening will add 5,500 new riders to the subway system -- residents in areas such as Cleveland Park who now drive to work. These commuters will now be able to walk to one of the three new stations.

Another 8,500 Maryland and District residents who now take the bus to work are expected to start switching to rail service, despite the fare increases. The steepest increases will come for residents of the Silver Spring-Bethesda and Rockville-Gaithersburg areas, who will pay $2.90 and $3.50 a round trip, respectively, under the new bus-rail downtown route, rather than the current $2.20 and $2.80.

Under current plans, bus service in the Connecticut Avenue corridor will be reduced with the elimination of the L1, L5 and L7 routes and reductions in the L2 and L4 service. But these changes have not been officially approved and will be the subject of public hearings over the next six months.

Metro spokesman Al Long said yesterday the six rail cars being added to subway service will come from the transit authority's maintenance reserve and will include two cars now used to gather revenue in the evening.

The first of the 94 rail cars on order in Italy are expected to arrive next June for assembly and testing. Yesterday, the board signed a nearly $200 million contract to purchase an additional 200 cars from the same manufacturer.

The board also announced yesterday that subway riders who journey downtown for Mall festivites on the Fourth of July will "pay 50 cents in the barrel, coming and going." In the past, subway transportation home has been free, but Metro officials said this costs the transit authority $50,000 to $100,000 and is a holiday gesture they no longer can afford.

Also at yesterday's Metro meeting, the board voted to restore more than $500,000 in cuts, most of it drawn from current fiscal year savings, to the next fiscal year's budget for computerized systems that will help the transit authority order spare parts to maintain the trains on a timely basis.

In related business, Long reported subway service on the Red Line was halted for about 25 minutes late yesterday morning after operators saw smoke in the tunnels between the Metro Center and Farragut North stations. The smoke was apparently caused when the motor of a fan in the tunnel burned out. Service was resumed when Metro and D.C. fire officials determined there was no fire in progress along the subway line.