Ovariectomized (OVX), hypothalamo/pituitary-disconnected (HPD) ewes were used to ascertain the short-term effects of estradiol on the number of gonadotropin-releasing hormone (GnRH) receptors in the pituitary gland. The time course of the study was such that measurements were made during the period of short-term negative feedback and positive feedback. Groups of 4 OVX-HPD ewes were given 250-ng pulses of GnRH each hour and an i.m. injection of oil (Group 1) or 50 micrograms estradiol benzoate in oil (Groups 2-4). Blood samples were collected from each ewe prior to treatment with estradiol or oil and again immediately before slaughter. Groups 2, 3, and 4 were killed 6, 16, and 20 h, respectively, after administration of estradiol. Amplitudes of luteinizing hormone (LH) pulses and average plasma concentrations of LH were reduced 6 h after estradiol treatment. Sixteen and 20 h after injection, the average plasma LH levels were elevated, but pulse amplitudes were similar to preinjection values. The number of GnRH receptors was significantly (p less than 0.01) increased within 6 h of estrogen treatment and further increased 16 and 20 h after treatment. Pituitary content of LH was similar in all groups. These data indicate that the number of GnRH receptors in the pituitary gland of ewes can be acutely influenced by a direct effect of estradiol. However, the magnitude and direction of the change in receptors number does not account for the changes in pituitary responsiveness to GnRH, suggesting estradiol also modifies post-receptor mechanisms that influence secretion of LH.