A heart surgeon who supervised more than two dozen operations performed by Cmdr. Donal M. Billig said yesterday that he told Bethesda Naval Hospital officials several times about his concerns over Billig's competence and was always assured that "something would be done."

Dr. Reginald Peniston, now head of cardiothoracic surgery at Howard University, testified for nearly six hours yesterday about his role in supervising Billig, now charged with the deaths of five patients at Bethesda. From January through June 1983, Peniston said, he operated about 30 times with Billig, who had not practiced heart surgery for about six years and was undergoing a retraining period at Bethesda.

In 1983, while still at Bethesda and later as a civilian, Peniston said he approached the chief of surgery to discuss his concerns with Billig's past and present work. "You've got a problem on your hands," Peniston said he told chief of surgery Capt. J. Raymond Fletcher after he left Bethesda in June 1983. Fletcher "assured me something would be done."

Peniston said yesterday that as the supervising surgeon he was responsible for Billig's actions during Billig's early operations, which both physicians attended, particularly one in March 1983 that resulted in the death of retired Air Force Lt. Col. Harold Coplan of Gaithersburg. But, he added, Billig often operated "too fast and with too much confidence" to be controlled easily.

"In the Coplan case he moved too fast before I could stop him," Peniston said. In future operations, he added, "The only way I could live with myself was to become a dictator. I never had to yell in the operating room before."

Billig, 54, is charged with culpable negligence in the deaths of five patients at Bethesda in 1983 and 1984 and 24 counts of dereliction of duty for operations he undertook in 1983 without supervision. In the Coplan bypass operation, Billig is charged with killing Coplan by improperly sewing and tying vein grafts, causing tears at those grafts and improperly repairing the tears.

Peniston, 37, said the operation, which he considered a low-risk case, became difficult after Billig attempted to graft a vein onto the patient's aorta. Billig pulled the threads of the suture too tight and cut the tissue, creating a hole that the two surgeons struggled to repair, Peniston said.

After futile attempts to close the hole, Billig suggested clamping off the blood flow to that area without cooling the heart, a method that Peniston said was an older technique. Peniston said he agreed to follow Billig in light of the fact that the older doctor was a certified heart surgeon.

That was a mistake that created other complications, Peniston said, and Coplan died on the operating table. Peniston admitted several times yesterday he regretted listening to Billig that day. "Doing that case, I felt I didn't follow my own judgment . . . looking back at that case, I regret it," Peniston said.