The Group Health Association and a union representing 160 physicians agreed late last night to extend their current contract for 24 hours and said bargaining would resume today aimed at averting a possible strike that would affect 145,000 members of the area's largest group health plan.

The extension was announced just as the current contract between GHA and the Capital Alliance of Physicians would have expired. It came at the end of a day of negotiating in a dispute that centers on physicians' workloads but also reflects the pressure on providers of health care to cut costs.

A union spokesman, in announcing the agreement to extend the three-year contract past its original expiration date of 12:01 a.m. today, declined to comment on any progress on the talks, which he said would probably resume "in the forenoon." The union voted 88 to 11 Friday night to authorize a strike. The Federal Mediation and Conciliation Service joined the talks yesterday.

"We are still hopeful of a settlement, but we have prepared for the worst," GHA executive director Robert Rosenberg said earlier yesterday. Mailgrams and telephone calls will be made today to explain emergency procedures to patients in case of a strike, he said.

Only emergencies, hospital care and childbirth will be handled by doctors at GHA's seven centers in the District, Maryland and Virginia if the strike occurs, and other medical visits will be postponed, GHA said.

The key dispute, according to both sides, is over doctors' work hours and shift schedules, with GHA seeking to increase office visit hours for many doctors and to change shifts to require more night work.

Officials of the doctors union contend that the changes would create "assembly line medicine" by leaving doctors less time with each patient and requiring them to speed up other duties such as their hospital rounds.

"We are not talking about assembly line medicine," Rosenberg said in an interview. "We have proposed modest changes" to assure that doctors spend a "reasonable minimum" amount of time on office visits and treat more patients.

Doctors work 35-hour weeks, and GHA wants to require between 20 and 35 of those hours solely for office visits, depending on the specialty. No such requirements exist now.

"You have to understand we are living in a very serious competitive environment, and our major issue is to provide access to medical service at a reasonable price," said Rosenberg, who has headed the 50-year-old, cooperatively owned association since 1982.

GHA is proposing a new "incentive" system to make doctors more productive, he said. GHA proposed that 5 percent of doctors' salaries be withheld in an escrow account, but then be repaid -- with a 5 percent incentive bonus -- if doctors groups are able to treat particular numbers of patients.

"This is not revolutionary. It is consistent with the way many physicians are being reimbursed" in the rapidly changing health care industry, said Rosenberg, a Harvard-educated pediatrician. GHA, facing stiff competition from an increasing number and variety of health care competitors, is also seeking changes in work shifts to better accommodate patients, he said.

Rosenberg said many Group Health members hold 9-to-5 jobs and can't get to doctors during the day.

Dr. Nieves Zaldivar, the union president, said, "The basic issue here is that we physicians insist on continuing to have enough time with each patient to assure quality medical care."

GHA doctors now work in the evening at three centers and are frequently required to work at nights when patients are hospitalized. Zaldivar said doctors are not disputing the need for night work but disagree with GHA's proposed scheduling method.

Doctors at Group Health earn an average of $91,000 a year, according to GHA figures, and receive 20 to 25 days of annual vacation, 13 holidays and generous benefits.

"Money is a background issue. The productivity program is the main concern . . . . The doctors hit the ceiling when they saw the Group Health proposals," said union spokesman Henry C. Fleisher.