The purpose of this study was to determine whether changes in the levels of neural progestin receptors are associated with mating-induced heat abbreviation in the rat. Mating caused a decrease, 8-10 hr after stimulation, in the duration of sexual receptivity. No difference in the concentration of cytosol progestin receptors was found between mated and nonmated animals 8-10 hr after treatment. Mating caused a statistically significant decrease in the concentration of nuclear progestin receptors at that time. However, this difference was probably not related to heat abbreviation, because no difference in nuclear progestin receptor levels was found between animals mated with vaginal masks and animals mated without masks, while mated-masked animals were found to exhibit significantly higher levels of sexual receptivity than mated-nonmasked animals after treatment. In addition, mating either with or without masks caused a statistically significant increase in serum progesterone levels relative to that measured in nonmated animals. Therefore, it is suggested that mating, even in the absence of cervical-vaginal stimulation, may cause the release of adrenal progestins which, in turn, may influence the rate at which neural progestin receptors accumulate and are processed within cell nuclei. The results of the present study suggest that mating-induced heat abbreviation in the rat is not associated with decreased levels of neural cytosol or nuclear progestin receptors.