In order to determine if the enhancement of sexual behavior in hysterectomized rats is associated with an increased level of hypothalamic nuclear progestin receptors, ovariectomized (OVX) and ovariectomized-hysterectomized (OVHX) rats were injected with 2 micrograms estradiol benzoate. Twenty-four hr later, animals were injected with 0.5 mg progesterone and were tested for sexual receptivity every 4 hr. Hysterectomy had an overall facilitatory effect on lordosis and increased the duration of the period of sexual receptivity by about 4 hr. In a second experiment, similarly-treated animals were killed 4, 8, or 12 hr after progesterone injection, and hypothalamic nuclear progestin receptor levels were measured. In contrast to what has been reported for guinea pigs, nuclear progestin receptor levels decreased to baseline 8-12 hr before the termination of sexual receptivity. Nuclear progestin receptor concentrations were higher in OVHX rats than in OVX rats at 4 hr after progesterone injection, and there was a trend toward higher receptor concentrations in OVHX rats at 8 hr. These results demonstrate that hysterectomy-induced facilitation of sexual receptivity is associated with an increased level of hypothalamic nuclear progestin receptors. Furthermore, they suggest a fundamental difference in the regulation of nuclear progestin receptor retention between rats and guinea pigs.