In these studies we examined the temporal parameters of estradiol (E2) priming required before progesterone (P4) amplifies the surge release of luteinizing hormone (LH) in short-term ovariectomized (OVX) rats. Thereafter, we correlated the time of appearance and concentrations of estrogen nuclear (E2Rn) and progestin cytosol (PRc) receptors in brain regions known to be involved in the surge release of LHRH. Steroid receptor concentrations were also measured in the pituitary gland. 1 week after OVX (day 0), Silastic capsules containing E2 (150 µg/ml in oil s.c.) were placed at 09.00 h. The serum E2 concentrations produced by these capsules peaked (41 ± 2.1 pg/ml) at 10.00 h on day 0 and declined thereafter to values which ranged between 19 and 10 pg/ml on days 1 through 4. Some of these E2-treated rats also received 2 P4-containing Silastic capsules (50 mg/ml s.c. in oil) at 09.00 h on either days 0, 1, 2, 3 or 4. Serum P4 concentrations produced by such capsules were 9.3 ± 0.5 ng/ml. Blood was collected sequentially at 09.00 h and at hourly intervals from 12.00–18.00 h on the day that the animals received the P4 capsules and 10.00 and 15.00 h samples also were taken from each group the next day. LH surges occurred in these E2-treated rats in the afternoons of days 2–4, but not on days 0 and 1. P4 treatment on day 1 elicited an LH surge and on days 2–4 it amplified plasma LH surge concentrations and advanced by 1 h the time of release of this gonadotropin. Separate groups of OVX rats were sacrificed before and at various times after E2 treatment and the concentrations of E2Rn and PRc were measured concurrently in the preoptic area (POA), medial basal hypothalamus (MBH), corticomedial amygdala (CMA) and the pituitary glands (PIT) of these rats. The E2Rn levels in these tissues paralleled the changes which occurred in serum E2. 1 hour after placing the E2 capsules (10.00 h day 0), E2Rn levels increased significantly compared to OVX controls, and thereafter they had stabilized by 15.00 h on day 1 and remained constant through 15.00 h on day 4. There was no E2 induction of PRc within 6 h after E2 treatment (10.00 and 15.00 h on day 0) in POA, MBH or PIT. Further, regardless of the duration of treatment, E2 did not induce PRc in the CMA. The first significant induction of PRc that we observed was at 15.00 h on day 1 (30 h after E2) and it then remained constant thereafter through 15.00 h on day 4 in POA, MBH and PIT. Pituitary responsiveness to 2 pulse injections of LHRH increased with the duration of E2 exposure, but additional P4 treatment did not further amplify LHRH-induced LH release on any of the days examined. Consequently, it requires 24 h of E2 priming for P4 treatment to amplify LH surges in short term OVX rats and this physiological effect of P4 directly correlates with the first significant E2 induction of PRc in POA, MBH and PIT (day 1). The data further emphasize that amplification of LH surges in E2-treated rats is not due to an increase in responsiveness of the pituitary gland to LHRH but rather, such P4 effects are mediated via brain mechanisms involved in the surge release