Washington's Metro subway will become a genuine regional rapid transit system for the first time Friday morning when area commuters find themselves with a whole new set of variations on how to get to work and how to pay for it.

At 5 o'clock trains will start running from both ends of Metro's new Blue Line, which links 18 stations on a 12-mile route between National Airport in Virginia and the RFK Stadium area in the District. Riders will be able to change trains at Metro Center to the existing Red Line, running between Dupont Circle and Rhode Island Avenue.

At the same time D.C. bus riders will get their first fare increase in seven years. The cost of a bus ride will go from 40 cents to 50 cents during Metro's so-called "peak" periods - between 6 and 9:30 a.m. and 3 and 6:30 p.m. Monday through Friday.

There are other changes for the region's transit riders:

Farecard, an electronic fare-collecting system, will become mandatory for subway riders. For three weeks, they have had the option of using either Farecard or the old, exact-change-in-a-farebox method.

Transfer procedures between train and bus will change drastically. It will no longer be possible to transfer from bus to train; the rider must pay two full fares. It will be possible to transfer from train to bus; the rider must get his transfer at the station where he boards the train. Because subway fares have been adjusted, the total round-trip cost for most Virginia and Maryland residents will be about the same as it is for a bus-only trip today. It will be noticeably higher for D.C. residents who travel in the rush hour, however, because of the higher bus fares.

Reduced fares will go into effect at all hours for elderly and handicapped citizens who have proper Metro identification. Until Friday, reduced rates were available only during the "off-peak." However, elderly and handicapped subway users who want the special rates will have to buy special farecards at Metro sales points to take advantage of the rates. The Farecard vending machines in Metro stations will not sell reduced-rate cards.

Major changes in the Metrobus system are scheduled in the near future as Metro's directors seek to cut total operating costs by turning the buses into feeders for the subway and by eliminating most parellel bus-rail service.

On July 17, most of the buses that serve North Arlington and northern and western Fairfax County, and most of the buses from Prince George's County and Southeastern Washington, will be rerouted to various Metro subway stations.

The Rosslyn station will be most heavily impacted with that change; with the influex of Virginia buses. The Prince George's County and southeastern D.C. buses will be rerouted primarily to the Stadium-Armory and Potomac Avenue Metro stations.

On Sept. 4, most of the buses that serve South Arlington, Alexandria and southern Fairfax County including the Shirley Highway corridor will be rerouted through the Pentagon. Many other routes will be terminated there and their riders forced on to the subway system.

But before the bus routes are formally changed, many commuters are expected to find ways to use the subway system, particularly those whose trips to work would be made much shorter in time by riding the train.

Another class of commuters - those who travel by auto, live in the southeastern sections of the metropolitan area and work downtown, will find relatively convenient parking near the Stadium-Armory Metro Station.

The No. 3 Parking Lot on East Capitol Street will be open for 1,200 cars for all-day parking starting Friday. The charge will be $1, which is $2 to $3 cheaper than downtown rates. The train from Stadium-Armory to Metro Center, at 12th and G Streets NW, will cost 45 cents. Parking will be free on Friday only to encourage commuters to use the lot.

But Stadium-Armory is the only station opening Friday that will welcome auto commuters. All the other stations are in heavily impacted business or residential areas and regional planners are very concerned about the additional traffic they think the Metro Stations will create.