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The effect of a subcutaneous injection of estradiol on the secretion of pituitary prolactin in the rat and the relationship between serum estradiol level and luteinizing hormone (LH) secretion in mare were reviewed. In addition, the effect of estradiol injection on LH secretion and the metabolism of [14C] estradiol in intact and bile duct fistulated pony mares were studied. Low (0.1 mug/day/rat) to moderate dose (5 mug/day/rat) of estradiol benzoate injected subcutaneously to mature or immature rats significantly increased pituitary content of prolactin and serum prolactin level five- to tenfold. On the other hand, high dose of estradiol (10 mug/day/rat or more) was less effective in stimulating prolactin secretion, and it appeared that progesterone injected concurrently with estradiol had some inhibitory action on the stimulatory effect of estradiol. Studies in pony mares showed that the physiologic level of serum estradiol during proestrus was important for the induction of the ovulatory surge of LH. Intramuscular injection of a low dose (2 or 4 mg/mare) of estradiol was stimulatory, whereas a high dose (8 mg/mare) was inhibitory for LH secretion in pony mares. Results of the estradiol metabolism studies indicated a relatively long half-life for estradiol in the mare. The majority of the [14C] estradiol metabolites appeared in the urine within 24 hr following intravenous injection. Enterohepatic circulation appeared to be important for estradiol metabolism in mare.