Gonadotropins were measured in young (3 months) and old (24 months) male Sprague-Dawley rats. Basal levels of LH were lower in old animals, whereas the basal levels of FSH were unchanged. The levels of gonadotropins increased significantly following castration in both age groups, However the old animals secreted less LH than the young. When 50 mg/100 g BW of testosterone propionate (TP) was given, the postcastration levels of FSH and LH were suppressed only in the old rats. When higher doses of TP were given, the levels of FSH and LH were reduced in both groups. However these levels remained suppressed longer in the old rat than in the young. The levels of FSH and LH were elevated after an iv injection of 10 or 100 ng gonadotropin releasing hormone (GnRH/100 g BW. FSH levels were similar in both groups of animals. LH values were higher in the young rats only after the injection of 100 ng GnRH/100 g BW. These data suggest that age differentially influences the control mechanisms for FSH and LH secretion.