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The ovarian progesterone secretion markedly increased on the evening of the day of proestrus during the estrous cycle in rats. This increase was completely disappeared when the rats were hypophysectomized at 5.00p.m. on proestrus. Intravenous administration of pituitary extracts obtained from the rats at any stage of the estrous cycle was able to restore the increase in progesterone secretion in the rats hypophysectomized at 5.00p.m. on proestrus. Various gonadotropins and other pituitary hormones were examined to see their effects on ovarian progestin secretion in the hypophysectomized rats. Luteinizing hormone (LH) showed a strong stimulating action which was relatively long-lasting and its minimum effective dosage appeared to be 0.2 to 0.5μg per rat. A similar stimulatory effect was produced by follicular stimulating hormone (FSH), human chorionic gonadotropin (HCG), pregnant mare's serum gonadotropin (PMS), thyroid stimulating hormone (TSH), and growth /hormone (GH), but not by prolactin, adrenocorticotropin (ACTH), oxytocin, and vasopressin. The action of FSH was transient and its potency was only 1 to 2 per cent of the activity of LH. Taking account of a minute amount of LH which possibly contaminates the preparations of FSH, TSH and GH, it is suggested that a preovulatory increase of progesterone secretion is most likely caused by LH released from the pituitary into the circulating blood.

Acute Effects of Various Gonadotropins and Other Pituitary Hormones on Preovulatory Ovarian Progestin Secretion in Hypophysectomized Rats