Abstract The purpose of this investigation was to determine whether any differences could be demonstrated in the ability of old versus young ovariectomized rats to release prolactin and LH in response to ovarian steroid administration. Female Long Evans rats 20–24 months of age and 4 months of age were ovariectomized. Fourteen days later, each rat was injected with estradiol benzoate, followed three days later by an injection of progesterone. Blood was collected from an intra-atrial cannula for eight hrs after progesterone treatment for radioimmunoassay of prolactin and LH. Basal plasma levels of prolactin were higher in the old than in the young rats. The young rats showed a marked rise in plasma prolactin and LH in response to progesterone, whereas the old rats exhibited no elevation in prolactin and a smaller increase in LH than in young rats. Changes in hypothalamic function in the old female rats are believed to account for the differences observed.