The role of estrogen and progesterone in facilitating the proestrous release of LH was examined in 4-day cyclic rats. Ovariectomy at 1000 hr diestrous day 2 eliminated the expected proestrous plasma LH surge. Administration of either 1.0, 10.0 or 50 μg estradiol benzoate (EB)/100 g body wt (1000 hr diestrous day 2) re-established gonadotropin release in these animals, with the intermediate dosage being most effective. However, peak plasma LH concentrations did not approach normal proestrous levels, and the rates of release were abnormally slow and more sustained. On the other hand, 10.0 μg 17β-estradiol (E2)/100 g body wt was more effective in eliciting the discharge of normal patterns and concentrations of pituitary LH. Progesterone (2 mg), administered at 0930 hr proestrous potentiated the plasma LH response in estrogentreated ovariectomized animals and also synchronized the timing of the gonadotropin surge. Estrogen replacement alone failed to produce a rise in plasma LH on proestrus in adrenalectomized- ovariectomized animals. When progesterone was administered to these rats following estrogen priming, elevated proestrous plasma LH concentrations were again observed. These results suggest that progesterone is required to act synergistically with estrogen to facilitate the proestrous surge of LH in the rat. The most likely source of this progesterone is the adrenal gland. (Endocrinology93: 694, 1973)