The present studies were designed to answer three questions: (1) how will a progressive increase in serum estradiol (E2) in ovariectomized (OVX) rats affect progesterone (P4)-induced luteinizing hormone (LH) surge concentrations? (2) Can steroid-induced LH surges be correlated with estrogen nuclear receptor (E2Rn) and progestin cytosol receptor (PRc) levels in brain regions known to regulate LH secretion, and (3) do differences in pituitary responsiveness to LHRH in E2- or E2P4-treated OVX rats parallel changes in E2Rn and PRc concentrations in this gland? 1 week after ovariectomy of adult cyclic rats (day 0), Silastic E2 capsules were placed subcutaneously at 09.00 h and produced serum E2 levels of 6∼8 (low), 12–19 (medium) and 27–37 (high) pg/ml, respectively. 2 days later (day 2), some rats also received Silastic P4 capsules subcutaneously which elevated serum P4 concentrations to 10–12 ng/ml. In rats with low serum E2, P4 treatment induced peak serum LH levels of 913 ng/ml. When serum E2 was increased to the medium or relatively high physiologic range, P4 treatment resulted in LH surge levels of 4,686 and 5,030 ng/ml. OVX controls and E2-treated OVX rats were sacrificed at 10.00 h on day 2 and E2Rn and PRc were measured concurrently in the preoptic area (POA), mediobasal hypothalamus (MBH), corticomedial amygdala (CMA) and pituitary gland (PIT). Raising serum E2 from OVX levels to the low range significantly increased both E2Rn and PRc in MBH and PIT, but not in the POA or CMA. When serum E2 was raised from the low to medium range, E2Rn and PRc increased in MBH, PIT and POA, but only E2Rn levels were significantly elevated in CMA. Elevations in serum E2 from the medium to high range significantly increased E2Rn in POA and CMA, and PRc only in PIT. Pituitary responsiveness to LHRHon day 2 was evaluated in E2 and E2P4-treated rats. LHRH-induced LH release increased with increasing serum E2 concentrations. However, P4 treatment neither amplified nor suppressed LHRH-induced LH release in any of these E2-treated rats. We conclude from these studies that only E2-induced increases in POA and MBH PRc may be correlated with P4-amplification of LH