Gary S. Mandel, 41, son of former Maryland governor Marvin Mandel, was convicted today of forging his doctor's name on four prescriptions for Dilaudid, a potent narcotic painkiller.

A U.S. District Court jury found Mandel guilty of four counts of forgery but acquitted him on four more serious counts of drug conspiracy and distribution.

Mandel, a Towson criminal lawyer, faces possible disbarment and up to four years in prison plus $30,000 in fines for each of the four forgery counts. Each of the other four counts carried a possible penalty of 15 years and $25,000 in fines. Mandel's attorney said he will seek a new trial on the forgery counts.

In an unusual move, jury members, some of them red-eyed, gathered on the court house steps after announcing the verdict and had their foreman read this statement to the media: "We believe the defendant was impaired due to medical addiction to Dilaudid . We hope the court will take that into consideration in sentencing."

The prosecution had charged that Mandel was an addict who pressured and manipulated a "negligent, gullible and spineless" doctor into prescribing 20,000 Dilaudid pills over a 2 1/2-year period from 1982 to 1984.

During that time, prosecutors said, Mandel sold pills to girlfriends and his law clients. When his doctor finally refused to prescribe anymore Dilaudid he wrote four prescriptions for himself, forging the doctor's signature, they said.

The defense maintained that Mandel became addicted to Dilaudid while in the care of Dr. Thomas N. Carter of Chevy Chase, to whom Mandel had gone to seek relief from chronic arthritis, back and neck pain.

Defense attorney Paul Kramer acknowledged that the prosecution was correct in claiming that Mandel routinely ground up the Dilaudid pills and injected them, and on at least one occasion injected himself in a pharmacy bathroom.

In testimony last week, Mandel said Carter had given him a prescription pad for emergencies and told him, "You write the prescription and I'll okay it."

"Those weren't forgeries," said Mandel after the verdict today. "Those were tragic years induced by a doctor who kept on writing prescriptions."

Mandel testified that during those years he lost his job with a Washington law firm and was so "deeply involved with myself and drugs . . . I was a recluse," leaving his Columbia home only to get Dilaudid.

Mandel said after the verdict he was pleased with the four acquittals and disappointed with the four convictions. "I think the jury got confused by having [the charges] all lumped together," he said.

Kramer had sought unsuccessfully to have the forgery counts tried separately from the distribution and conspiracy counts.

Carter, who is not implicated in the case, testified that Mandel "always had some story" about why he needed more Dilaudid prescriptions, sometimes saying he'd lost his pills or that a pharmacy did not have enough of them on hand to fill his entire prescription.

Carter said that although he wrote 340 Dilaudid prescriptions for Mandel -- about 20,000 pills -- he became aware only in late 1983 that Mandel had become addicted to them. He said he believed the pills were necessary to "control the pain" from Mandel's arthritis.

Carter denied giving Mandel a prescription pad or the authority to sign his name.

Marvin Mandel testified that even after his son went through treatment at a pain clinic in 1984 to help him overcome his addiction, Carter continued to prescribe Dilaudid for the younger Mandel. Gary Mandel's mother, Barbara Mandel, testified that she gave her son $50,000 for living expenses between 1982 and 1984.

Barbara Mandel said after the verdict, "I should have given him another name." She argued that the government singled her son out for prosecution because he is a Mandel.

Two former girlfriends of Gary Mandel testified, in return for immunity from prosecution, that he sold Dilaudid for $20 a pill to a former client many times in 1982. Mandel denied ever selling the drug.