The son of former Maryland governor Marvin Mandel was indicted by a federal grand jury today on charges of conspiring to distribute narcotics, using forged prescriptions to obtain narcotics and distributing narcotics.

The eight-count indictment charges that Gary Steven Mandel, 40, a Baltimore lawyer, conspired with a Middle River, Md., woman, Katherine Ann Harris, to obtain physician prescriptions enabling him to acquire thousands of tablets of Dilaudid, a narcotic painkiller.

The indictment also charges that Mandel, a graduate of the University of Maryland law school, used prescriptions from physicians and forged prescription forms to obtain Dilaudid tablets from pharmacies in the Baltimore-Washington area.

Mandel and Harris "distributed quantities of these Dilaudid tablets to others in return for money and other consideration," the indictment charges.

Mandel's lawyer, Paul R. Kramer, said that his client "had become clinically addicted on Dilaudid" when he took the drug under a doctor's care after "a series of very painful back operations" during the late 1970s.

"All medication he got was pursuant to authorization of his doctor, and he denies any sale or distribution of these pills," Kramer said. "They are gotten for his own addiction or his own needs."

Kramer, who termed the indictment "an unfortunate situation," described Harris as a "former friend" of Mandel.

Dilaudid, a highly addictive drug taken orally or injected, is a Schedule II narcotic under federal law, meaning that it is one of a class of drugs -- including morphine, opium, and Demerol -- that can be prescribed for medical purposes but that present the highest risk of abuse next to Schedule I narcotics, such as heroin, which are deemed to have no legitimate medical use.

The drug is commonly used by addicts as a substitute for heroin, and can have a street value of $40 for a 4-milligram tablet.

Mandel, contacted by telephone this afternoon at his law office, said he had not heard of the indictment and declined to comment on it.

The alleged conspiracy, according to the indictment, lasted from August 1982 through last September.

Besides the conspiracy count, the indictment charges Mandel with three counts of distribution of Dilaudid and four counts of using a forged prescription to obtain the drug.

The four forged prescriptions alleged in the indictment, dated between Aug. 19 and Sept. 12 and for a total of 118 tablets, were written on forms bearing the name of a Chevy Chase physician.

The maximum penalty for conspiring to distribute or distributing Dilaudid is 15 years in prison and a $25,000 fine. The maximum penalty for obtaining the drug by means of a forged prescription is four years in prison and a $30,000 fine.