Heroin use was consistently associated with low plasma testosterone levels in narcotic addicts. Heroin addicts maintained on high dosage methadone (80-150 mg/day) also had depressed testosterone levels. Patients on low dosage methadone maintenance (10-60 mg/day) had testosterone levels which were not significant;y different from normal adult male controls. An inverse relationship between methadone dosage and plasma testosterone occurred during methadone detoxification. These findings indicate that heroin and methadone alter male androgen levels with possible derivative effects upon sexual and aggressive behaviors.