Changes in sexual function and hormone levels are commonly found in subjects addicted to narcotics. In this study we examined 16 male and 3 female addicts who had been taking heroin (H) in the last year in doses higher than 150 mg/day. In these patients, who presented similar clinical problems, we assayed by RIA the plasma levels of heroin, testosterone, (T), dihydrotestosterone (DHT), androstenedione (A), luteinizing hormone (LH) and follicle-stimulating hormone (FSH) for periods of 150 min, 6 h and 9 h. We found a significant reduction of T and DHT concomitant with higher plasma concentrations of heroin but no relevant changes of A, LH and FSH. T and DHT returned to the initial levels after the decrease of heroin concentration. The GnRH test effectd on a female subject allowed us to make the diagnosis of hypothalamic amenorrhea. In the same patient no circadian rhythms for T, DHT and A were detected.