Female guinea pigs are more sensitive than males to the lordosis-promoting effects of sequential estradiol-progesterone treatment. This study explored a possible cellular basis for this reduced sensitivity. Cytoplasmic progestin receptor concentrations were determined using a [3H]R5020 binding assay for a dissection of brain tissue which included the hypothalamus, preoptic area and septum (HPS) from male and female guinea pigs 40 h after administration of estradiol ben-zoate (EB). Male guinea pigs injected with 10 μg EB have a progestin binder in cytosol from HPS and cerebral cortex that is similar to that in females on the basis of apparent dissociation constant and steroid specificity. Injection of either 1.6 or 10 μg EB caused a larger increase in the concentration of cytoplasmic progestin binding in female HPS compared to the same dose in males. A 10 μg EB injection induced a concentration of progestin binding in male hypothalamus not significantly different from that induced in female hypothalamus by a 1.6 μg EB injection (a dose that consistently facilitates sexual recepivity in females when followed 40 h later by 0.5 mg progesterone). Despite the induction by EB of a behaviorally-sufficient concentration of cytoplasmic progestin receptors, fewer males (3/10) than females (8/10) responded to EB (10 μg)-progesterone treatment with the expression of lordosis. Finally, using a nuclear progestin receptor [3H]R5020 exchange assay, we found that male guinea pigs injected with 10 μg EB accumulated a slightly lower concentration of progestin receptors in HPS cell nuclei 4 h after a 0.5 mg progesterone injection than similarly-treated females. Therefore, male guinea pigs show decreased induction of cytoplasmic progestin receptors in HPS after estradiol injection, and a decreased cell nuclear accumulation of these receptors after progesterone injection compared to females. We conclude that this sex difference in the progestin receptor system is insufficient to account, in itself, for the sex difference in behavioral sensitivity to estradiol and progest