There is virtually no information on the psychobiology of hypoactive sexual desire (HSD) in men. This study compares pituitary and gonadal hormones and nocturnal penile tumescence (NPT) during sleep in 17 physically healthy men with HSD and 17 age-matched, nondysfunctional volunteers. There were six HSD men with problems of sexual arousal and 11 with secondary erectile impotence. The subjects, aged 27–55, were studied in a sleep laboratory for 4 nights with the last night devoted to sequential blood sampling every 20 min. The total group of HSD men had significantly lower plasma testosterone, measured hourly through the night, than controls, and there was a positive relation between testosterone and frequency of sexual behavior. There were no differences in free testosterone, prolactin, luteinizing hormone (LH), and estradiol between the HSD and control groups. The NPT parameters of HSD men with secondary impotence were consistently and significantly lower than the nondysfunctional men. These data are discussed with respect to the role of gonadal hormones in male sexual function, the significance of NPT as a window to central biologic events, and to possible heterogeneity of men with hypoactive sexual desire.