Circulating levels of lh have been determined in intact and ovariectomized ewes before, during and after progesterone treatment. Neither 20 nor 40 mg progesterone daily for 10 days decreased the serum lh concentration during treatment. Following withdrawal, elevated levels of lh occurred on Days 4 to 6 in both intact non-cycling and ovariectomized ewes. Intact ewes treated during the initiation of the breeding season showed depressed levels of lh and failed to respond to progesterone treatment. The injection of 40 mg of progesterone daily did not delay the time of occurrence of the peak in lh activity.