• This was a double-blind comparison of methadyl acetate and two dose levels of methadone hydrochloride in the maintenance of 430 street heroin addicts from 12 Veterans Administration hospitals. The starting sample consisted of 146 patients receiving low-dose methadone, 142 patients receiving high-dose methadone, and 142 patients receiving methadyl acetate. Patients were first given 30 mg of both drugs, and doses were incremented by 10 mg/week until they stabilized at methadyl acetate, 80 mg three times a week, and methadone hydrochloride, 50 mg daily or 100 mg daily. Dosage was fixed for the balance of the 40-week treatment period. Safety was evaluated by clinical and laboratory observations conducted at frequent intervals throughout the study. Relative efficacy was evaluated by illicit drug use, program retention and attendance, and global staff judgments. It is concluded that methadyl acetate is as safe a drug as methadone and that it compares favorably with highdose methadone in terms of efficacy. Both methadyl acetate and high-dose methadone appear to be better maintenance regimens than low-dose methadone under the conditions of this study.