In sheep, physiological levels of estradiol and progesterone each suppress the pulses of LH characteristics of tonic LH secretion, but do so by completely different mechanisms. Estradiol treatment decreases LH pulse amplitude but not frequency and also inhibits the height of the LH peak resulting from the administration of gonadotropin-releasing hormone (GnRH). In contrast, progesterone decreases the frequency of LH pulses without reducing their amplitude or the response to exogenous GnRH. This suggests that progesterone suppresses tonic LH secretion by acting in the brain to decrease the frequency of GnRH pulses, while estradiol may suppress the response of the pituitary to GnRH and thereby decrease LH pulse amplitude.