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 micrograms/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.(ABSTRACT TRUNCATED AT 250 WORDS)