Pituitary content and concentration of LH and FSH were measured by radioimmunoassay from birth to adulthood. The pituitary contents and concentrations of both gonadotropins increased from birth to 70-90 days, then reaching adult levels. 1 µg/100 g body weight of synthetic LHRH was administered to immature (25 and 55 days of age) and adult male rabbits. LH and FSH were measured in plasma samples taken just before and 30 min after LHRH injection. At all stages studied, LHRH significantly increased circulating LH and FSH. The FSH response at 55 days and at adult age was lower than that of LH. There was a peripubertal increase of the mobilizable pituitary reserve of LH while the FSH reserve slightly but not significantly increased. This change in the pituitary responsiveness to LHRH could play a role in pubertal processes.