Gonadal function was elevated in 80 male heroin and/or methadone addicts by measuring basal plasma levels of follicle stimulating hormone (FSH), luteinizing hormone (LH), prolactin (PRL) and testosterone. In 41 subjects semen analyses were also undertaken. Three groups were distinguished consisting of 15 heroin addicts, 42 undergoing methadone treatment but continuing to take heroin, and 23 taking only methadone. All patients had normal plasma levels of FSH, LH and testosterone. Prolactin levels were normal in all subjects except for the 15 heroin addicts, in whom they were significantly higher than in controls (P less than 0.025). Semen analyses from all of the heroin addicts and from the dual heroin-methadone users were abnormal, whereas only 10 out of 22 (45%) of the methadone takers were pathological. In all cases asthenospermia was one of the abnormalities (100%). Twenty-four per cent also showed teratospermia and hypospermia and 17% showed oligozoospermia. Such seminal pathology, especially of forward motility, even in combination with normal hormone levels, might be an early indication of heroin toxicity to the male reproductive tract.