Effects of Altering the Balance Between Prolactin and Ovarian Hormones on Initiation of Lactation in Rabbits1

Recently we reported (Meites and Sgouris, 1953) that the ability of a moderate dose of prolactin to initiate milk secretion in rabbits with well developed mammary glands, could be effectively inhibited if optimal, mammary growth-promoting doses of estrone and progesterone were injected at the same time. In the absence of either one or both of the steroid hormones, this dose of prolactin elicited a good degree of lactation. The present experiment was undertaken to determine the lactational response in rabbits to injections of (a) large doses of prolactin during treatment with a constant but optimal combination of estrone and progesterone for mammary development and (2) a moderate dose of prolactin in the presence of less than optimal amounts of the two steroid hormones. EXPERIMENTAL Thirty-six New Zealand White rabbits, weighing 5 to 6 pounds each, were ovarieetomized and permitted 10 to 14 days recovery to remove the influence of the ovarian hormones.