The pulsatile pattern of LH secretion was studied in six patients with primary testicular failure and six healthy adult men by drawing peripheral blood samples every 10 min for 12 h. Mean basal LH concentrations were increased approximately 6-fold in hypogonadal men. In these patients mean absolute LH pulse amplitude was 3 times that of the controls [106 ± 27 (se) ng/ml vs. 31 ± 2 ng/ml; P < 0.01] and the frequency of spontaneous LH secretory episodes was increased 2-fold (between-pulse interval of 53 ± 3 min vs. 107 ± 8 min; P < 0.01). Mean pulse amplitude was directly related to mean LH concentration in hypogonadal men (r = 0.94, P < 0.01), but notin controls (r = 0.38, P = NS). Within each group there was no relationship between circulating testosterone or mean LH levels and LH pulse frequency. This study demonstrates that the increased circulating levels of LH in hypogonadal men are a consequence of an increase in both LH pulse frequency and pulse amplitude. The prolonged, more frequent sampling protocol used revealed a pattern of LH secretion not evident in previous studies. Finally, these data provide further, albeit indirect, evidence that androgens may regulate the intermittent release of LHRH in man.