The objective of this study was to determine the influence of estradiol-17β and/or progesterone on gonadotropin secretion at the level of the pituitary. Female rats in which the hypothalamo-hypophyseal connections had been permanently interrupted after castration served as the experimental model in which the effect of estradiol and/or progesterone on LH-RH-induced gonadotropin release was examined. In our experimental animals, LH secretion was readily activated by LH-RH administration. LH release was greatly augmented by the prior administration of estradiol benzoate (1 µg/kg b.w./day). Progesterone (5 mg/day) in the absence of estradiol did not modify the 10-min response to LH-RH but reduced the enhancement of LH secretion caused by estradiol pretreatment. Our findings suggest that estradiol potentiated the releasing effect of LH-RH at the level of the gonadotroph, whereas progesterone interfered with the potentiation effect. Plasma levels of FSH were not significantly elevated above the basal value by the administration of LH-RH alone, or in combination with estradiol and/or progesterone.