Physicians working for the Group Health Association voted last night to go on strike today against the association, which serves 145,000 members and is the Washington area's largest group health plan.

A spokesman for the physicians said the strike formally began at 12:01 a.m. today and that picketing would begin at 7:30 a.m. The physicians' contract had been scheduled to expire at 12:01 a.m. yesterday but was extended for 24 hours at the request of a federal mediator.

GHA had already postponed many nonemergency medical appointments scheduled for today while bargaining over a new contract between the association and the physicians' union remained deadlocked. The dispute revolves primarily around physicians' workloads.

A spokeswoman for GHA said the organization was planning to use about 30 supervisory physicians to staff its seven centers in the event of a strike by the 160-member union.

The strike is the second by the union, one of a small number of physicians unions in the nation. A 1977 strike by the group lasted for 11 days.

The union has said it will help with emergencies, hospitalized patients and childbirth but will cancel other duties in a strike.

Although the physicians voted 90 to 20 to begin striking, the negotiations between GHA and the union, the Capital Alliance of Physicians, were still under way early this morning, according to union spokesman Henry Fleisher.

However, GHA spokeswoman Ruth Jordan said shortly before 2 a.m. that negotiations had broken off early this morning.

"We are very disappointed," Dr. Robert Rosenberg, GHA's executive director, said in a statment.

"We felt certain tonight that we were moving toward a settlement," Rosenberg said in the early morning statement.

Earlier last night, Fleisher, the union spokesman, had characterized the outlook for a rapid settlement as not encouraging, saying that "there has not been much give-and-take to make the union people happy" about the progress of the talks.

Central to the dispute has been GHA's plan to increase the required number of office hours for many doctors, a move the union said would require doctors to meet "quotas" of patient visits and speed up service.

Managers of the member-owned GHA have tried to initiate a "productivity" system that would withhold 5 percent of salary if physicians fail to attain certain goals but give a 5 percent bonus if they reach the goals.

In a statement issued early this morning Dr. Nieves Zaldivar, president of the union, said many of its members "expressed deep resentment at the affronts to their professional ethics and standards contained in in GHA's proposal for arbitrarily limiting the time we spend with our patients . . . . "

Zaldivar, a pediatrician at GHA's Marlow Heights center, said the physicians also objected to a "patently unfair and unreasonable incentive system that is inimical to medical care of decent quality for GHA patients."

Doctors at GHA earn an average of $91,000, according to GHA figures, and the union spokesman has said money is not the main issue.

In his early morning statment GHA executive director Rosenberg said that in the last hours of bargaining "there were substantial changes in both sides' position."

GHA spokeswoman Jordan said the association had hoped to continue bargaining in the hours before the scheduled 7:30 a.m. start of picketing.

"We did want to continue working through the night," she said. "We felt we were so close."

She said the association is "ready to go back into negotiations, ready to work for a settlement," as soon as talks are reconvened.

GHA staff employes called hundreds of patients yesterday to determine whether today's appointments could be deferred. Jordan said many patients "were very understanding" and agreed to postponements.