Serum follicle stimulating hormone (FSH) and luteinizing hormone (LH) before and after intravenous injection of luteinizing hormone releasing hormone (LHRH) were studied in 71 male and female subjects of various ages. Mean basal FSH and LH levels were not significantly different in the male groups, except that FSH was significantly higher in the very old subjects. Postmenopausal female subjects had much higher concentrations of both hormones than had premenopausal women. LH was higher in female subjects shortly after the menopause than in very old female subjects. The serum concentrations of both FSH and LH 30 min. after intravenous injection of 200 mug LHRH were not different from the 60 min. values. There was no significant difference in the response of LH in the male groups. The peak FSH concentration was higher in the very old male subjects. Postmenopausal women had a much higher peak concentration of both FSH and LH than had younger subjects. The increment of LH, but not of FSH, was larger in female subjects shortly after the menopause than in very old female subjects. In both sexes there was a significant correlation between the basal FSH/LH ratio and age. In younger male subjects there was a close positive correlation between basal LH and serum testosterone, in older male subjects this correlation was negative and significant.