The Relationship Between the Rate of Testosterone Infusion and Gonadotropin Secretion1

Prepubertal and young adult male rats were castrated and continuous intravenous infusions of testosterone were administered for 2–3 days. Blood samples were obtained at various intervals and serum concentrations of FSH and LH were determined. In 43-day-old males a dose of approximately 57 μg/day was required to suppress serum LH to levels observed in intact controls, and FSH was also suppressed at this dose. Within one day after termination of infusions serum LH and FSH concentrations had returned to (or exceeded) the levels observed in castrates infused with vehicle only. This was true even after a very large dose of testosterone had been administered (918 μg/day). In 59-day-old males serum LH was suppressed somewhat by 40 μg testosterone/ day and further by 200 μg/day. FSH did not appear to be suppressed as readily in these rats as compared to the younger animals. Further experiments in which castrated rats were injected with testosterone or testosterone propionate revealed marked differences in the mag...