A major Metrobus route that serves downtown Washington, Georgetown and Arlington will be renumbered and will begin a new schedule starting today. a

The M-6 bus line, which has operated between Union Station and the Rosslyn subway station, is now called the No. 38 line. Instead of terminating at Rosslyn, it will run along Wilson Boulevard in Arlington to the Ballston Terminal of Metro's newly extended Orange Line. Between Rosslyn and Ballston, it will replace the No. 1 busline.

The change is one of 70 in Metro's bus service that begin today and tomorrow, chiefly affecting feeder bus service in Virginia to the Orange Line. Many bus lines, including No. 1, that started in the outer suburbs and went to Rosslyn will be cut back at Ballston.

The M-6 line was established in 1977 when the subway was opened under the Potomac River between Washington and Arlington. Starting at Union Station, it had provided service on Capitol Hill, Pennsylvania Avenue, 14th Street NW, K Street to Washington Circle and on M Street through Georgetown, crossing Key Bridge to Rosslyn.

The extended No. 38 line will pass within one block of every Orange Line subway station between Metro Center and Ballston, and will continue to serve Georgetown. No. 38 will be operated seven days a week, unlike the M-6, which did not run on Sundays and holidays.

The No. 38 will divide into two branches near Ballston, No. 38-A will run from the Clarendon subway station to Ballston via Wilson Boulevard and No. 38-B will run via Washington Boulevard.

Buses headed toward Washington marked 38-F will terminate at Farragut Square instead of Union Station.