Intravenous administration of synthetic LH-RH in doses of 100 μg resulted in a significant rise in serum LH and FSH without changing GH, TSH and cortisol in normal subjects, 33 men (group I, aged 18–29, group II, aged 30–49 and group III, aged 50–70) and 21 women (group IV, normal menstrating women aged 13–47 and group V, post-menopausal women aged 47–56). The dilution curves using serums with peak values of lh and fsh were parallel with standard curves by radioimmunoassay. After administration of LH-RH, the maximum level of LH was observed at 30 min in groups I–IV and 60 min in V and that of FSH at 60 min in all groups. There was a great degree of variation in individual response to LH-RH. The day-to-day variation was less in men than in women. In women, the basal levels, peak values and maximum increments above baseline of LH and FSH were statistically higher (p < 0.001) in the older group (V) than those in the younger group (IV). Similar results were obtained in men, although the maximum increments of LH and FSH were not different in regard to age. The ratio of FSH and LH levels decreased promptly and reached minimum at 30 min after administration of LHRH and then recovered. The mean (± sd) ratio of basal FSH and LH (F/L ratio) [0.74 ± 0.44 (I), 0.81 ± 0.71 (II), 1.02 ± 0.42 (III); 0.97 ± 0.54 (IV), 1.21 ±0.40 (V)], the mean (±sd) ratio of maximum increments of FSH and LH (ΔFSH/ΔLH ratio) [0.08 ± 0.03 (I), 0.10 ± 0.03 (II), 0.12 ±0.07 (III); 0.12 ± 0.07 (IV), 0.33 ± 0.08 (V)] and the ratio of circulating FSH and LH levels at 30 min after LH-RH administration [0.15 ±0.05 (I), 0.18 ±0.09 (II), 0.25 ± 0.11 (III); 0.20 ± 0.10 (IV), 0.59 ± 0.09 (V)] tended to be higher in older groups.