Thousands of Northern Virginia bus riders will be looking for new routes or new rides next year if service cuts proposed by Metrobus are adopted.

The service reductions and adjustments to 42 suburban bus routes, scheduled to begin in January, have been proposed by Metrobus in response to request by Fairfax and Arlington counties and the City of Alexandria to reduce the amount of subsidies those jurisdictions now pay for Metrobus service.

Metrobus officials estimate that adoption of all the route changes would save $800,000 a year. Those savings would be split almost equally among the three jurisdictions. But officials not that in the past only one-quarter to one-third of proposed route changes have actually been adopted.

Three public hearings on the proposed changes are scheduled this month and Metro officials predict spirited opposition to some of the changes, particularly those that affect rush-hour commuters.

Route 6, 8, 15 and 21, for instance currently originate in Northern Virginia and end in downtown Washington. If the proposed cuts are adopted, those routes would end at the Pentagon. Commuters continuing into the district would have to transfer to the subway or other intown buses.

"I'd sayd that would probably generate substantial testimony," said Millard (Butch) Seay, Metro's bus operations specialist. "It's inconvenient . . . but if you take all the routes that currently provide that type of service, they are underutilized."

A more drastic cut, though one that will affect fewer riders, is the elimination of many night routes. The majority of routes serving Alexandria, including 6, 7, 9, 10, 11, 12, 14 and 15, are scheduled for reduction or elimination after 9 p.m.

Many night riders are low-paid workers such as domestics or night watchmen who are most "transit dependent." Those riders are least able to afford alternate means of transportation, admit officials. But in any equation designed to save the most money while inconveniencing the fewest number of people, late night service always appears as an answer.

In Arlington, there is a proposal to eliminate midday service and service going into the District during rush hour on Route 16U and 16X. Those two lines serve neighborhoods between Shirlington and the Pentagon.

Another proposed adjustment would change Route 38, which now runs between the Ballston subway line and Union Station. If approved, the new route would leave Ballston, cross into Georgetown and continue north to Glover Park in the District.

Three public hearings on the proposed changes are scheduled for this month. The first will be on June 17 at the Alexandria Council Chambers. On June 18, a hearing will be held at the Arlington County Courthouse. The final hearing will be June 24 at West Springfield High School. All hearings begin at 7:30 p.m.

The following is a list of routes where changes and proposed and the areas affected. To obtain the exact changes proposed for a particular route, contact the Washington Metropolitan Area Transit Authority (637-1092).

Routes 1B, 1M, 1W, serving Heritage Mall, Annandale, Fairfax Hospital, Jefferson Village, Seven Corners, Dominion Hills and the Ballston Metro Station.

Routes 4A, 4B, 4E, 4H, serving Jefferson Village, Seven Corners, Arlington Boulevard, Arlington Forest, Pershing Drive and Rosslyn.

Routes 6A, 6B, 6F, 6L, serving Landmark, Brookville, downtown Alexandria, Bradlee, South Fairlington, Park-fairfax, Shirlington, Pentagon Metro Station, Federal Triangle and Potomac Park.

Routes 7A, 7F, 7M, 7X, serving Landmark, Lincolnia, Southern Towers, North Fairlington, Shirlington, Pentagon Metro Station, Federal Triangle, Southwest Terminal and Union Station Visitor Center.

Route 8M serving Shriley Duke Seminary Valley, Brookville, Pentagon Metro Station and Southwest Terminal.

Route 9C serving Fort Belvoir, Groveton, Belle Haven, downtown Alexandria, National Airport, Crystal City and Federal Triangle.

Route 10S serving Hunting Towers, Del Ray, Arlandria, Shirley Park, Shirlington, Westmont, Buckingham, Ballston Metro Station and Arlington Hospital.

Routes 11A, 11B, 11E, 11W, 11X, serving Mount Vernon, Fairfield, Rose Hill, Beacon Hill, Belle Haven, Hunting Towers, downtown Alexandria, National Airport, Federal Triangle.

Routes 12A, 12B, 12E, 14A, 15A, 15B, serving downtown Alexandria, Rosemont, Braddock Heights, Del Ray, Arlandria, South Arlington, Pentagon Metro Station and Federal Triangle.

Routes 16U, 16X serving Shirlington, Fort Bernard Heights, Westmont, Columbia Pike, Navy Annex and the Pentagon Metro Station.

Routes 18B, 18C serving Burke Centre, Rolling Valley Mall, Orange Hunt, Rolling Valley, Springfield, Springfield Mall, Landmark and Pentagon Metro Station.

Route 21C, serving Quaker Lane, Duke Street, Landmark, Pentagon and Federal Triangle.

Routes 22A, 22B, serving Walker Chapel, Ballston Metro Station, Buckingham, George Mason Drive, Four Mile Run Drive and Shirlington.

Routes 23M, 23N, serving McLean Hamlet, the CIA at Langely, McLean, Ballston Metro Station, Glebe Road and Shirlington.

Routes 24A, 24B, serving Tysons Corner, Falls Church, Ballston Metro Station, Clarendon Metro Station, Washington Boulevard and Pentagon Metro Station.

Routes 25A, 25B, serving downtown Alexandria, Landmark, Alexandria Hospital, Southern Towers, Northern Virginia Community College, Skyline Center, Northern Virginia Doctors Hospital and Ballston Metro Center.

Routes 38A, 38B, 38F, serving Ballston, Clarendon, Rosslyn, georgetown and Glover Park.