Effect of progesterone on plasma luteinizing hormone activity

The luteinizing hormone (LH) activity of ovariectomized rat plasma was estimated by the ovarian ascorbic acid depletion technique. A single subcutaneous injection of 25 mg of progesterone produced a slight decrease in this LH activity which was first demonstrable 2 days after injection of progesterone. If a second injection of 25 mg of progesterone was given 2 days after the first, a marked decrease in LH activity was found on testing a day later. Three daily subcutaneous injections of 5 mg of progesterone failed to significantly depress the LH activity in ovariectomized rat plasma. Pretreatment of these ovariectomized rats with estradiol benzoate rendered them more sensitive to the effects of progesterone in that 4 mg progesterone/day was then able to significantly depress the LH activity in their plasma. It was concluded that progesterone alone has a feeble inhibitory effect on the secretion of LH but that estrogen pretreatment sensitizes the animal to this inhibitory action of progesterone.