The neural substrate for estrogen (E) and E plus progesterone (P)-induced sexual receptivity was examined in 3 classes of adult rats: adult females ovariectomized for 11 days, and females and males gonadectomized on the day of birth. E responses were found only after implantation in the preoptic or medial hypothalamus; adult females had the largest fraction of responders. The addition of P significantly increased the fraction of adult and neonatally ovariectomized females responding when E implantation was made throughout the hypothalamus from preoptic to mammillary bodies. No E-P synergistic effects were seen in the day-1 gonadectomized males. E-P synergistic effects were found in the amygdala of the adult ovariectomized females. However, midbrain implants of E were ineffective for facilitating E-P-induced receptivity in the 3 classes of animals studied.