Group Health Association and its 160-member physicians union reached a tentative agreement late last night to end the bitter 23-day doctors' strike that has affected care for the 145,000-member health maintenance organization, according to federal mediators.

Capital Alliance of Physicians will vote tonight on a proposed three-year contract, which also must be approved by the nine-member board of the member-owned nonprofit GHA, the region's oldest health association.

The breakthrough came during the third consecutive late-night bargaining session at the Federal Mediation and Conciliation Service headquarters, where the two sides reached a compromise on the last remaining issue: the question of whether GHA will discipline nine to 11 doctors accused of neglecting patients during the strike.

Neither side would discuss details of the proposed settlement. "This has been so topsy-turvy, so sensitive that I don't want to louse anything up by saying anything about the agreement," a union spokesman said early today. A GHA spokesman also declined to comment pending approval of the settlement.

The strike, which has caused thousands of patients to postpone appointments or seek care at other facilities, could end as early as tonight, and GHA would immediately begin attacking a huge backlog of postponed appointments and complaints, officials said.

The strike began March 3 in a dispute over a GHA plan to increase productivity by requiring doctors to spend several more hours of their 35-hour week on office visits. The plan would have started an incentive pay system providing a 1 to 4 percent bonus above their $91,000 average salaries, depending on the ability of groups of doctors to treat certain numbers of patients.

Those proposals were strongly criticized by doctors who said GHA's plan, which was later modified, would result in speeding up care and leaving doctors less time for hospital visits and other duties.

GHA vigorously disputed the doctors' contention that the plan would create "assembly line medicine" and management contended that its productivity goals would bring GHA into line with others in the fast-growing HMO field. GHA director Dr. Robert Rosenberg said an "incentive reward" system was important to making GHA more competitive with the 15 HMOs expected to be operating in the Washington area by 1987.

Sources said yesterday the compromise settlement on productivity involves setting up a labor-management committee to study the issue. An impartial arbitrator would decide, if necessary, whether to establish the incentive pay system.

The final roadblock to the settlement involved GHA's accusation that some doctors "violated their professional responsibilities" by failing to honor a pre-strike agreement to treat emergencies and to handle scheduled surgery and other duties during the strike.

GHA served notice during bargaining that it plans to suspend or reprimand nine to 11 doctors for various alleged offenses.

The union contended that the incidents were exaggerated and contested the method GHA proposed for disciplining doctors. Sources said the proposed settlement likely will involve arbitrating the discipline cases, but no details were available.

During the strike, GHA kept open its nine health centers in Maryland, Virginia and the District, staffing the centers with management doctors, temporary replacements and 10 to 20 doctors who crossed picket lines to work.