Juvenile female guinea pigs rarely display estradiol-induced or progesterone-facilitated sexual receptivity. This experiment was designed to test the hypothesis that endogenous opiate peptides tonically inhibit the display of steroid-induced lordosis in juvenile females. Hartley guinea pigs were ovariectomized (OVX) at 10-11 days of age and treated with estradiol benzoate (EB, 10 micrograms) 5-7 days later. Injection of the opiate antagonist, naloxone (2.0 mg/kg), did not facilitate lordosis (compared to vehicle) in EB-primed, OVX juveniles; when these animals were subsequently given progesterone, and the treatment groups were reversed (i.e., the group previously treated with vehicle received 2.0 mg/kg naloxone 4.5 hr after progesterone, and vice versa), four of six naloxone-injected animals displayed lordosis, compared to one of six control animals. In a second experiment, a range of doses of naloxone was administered to EB-plus progesterone-treated OVX juveniles. Naloxone at the 2.0 mg/kg dose resulted in a significant increase in the percentage of females displaying lordosis, with lower doses (0.5-1.0 mg/kg) producing responses intermediate between those observed in animals receiving the 2.0 mg/kg dose and the vehicle. Furthermore, the facilitatory effect of naloxone on sexual receptivity was completely blocked by concomitant injection of morphine (10.0 mg/kg). These data suggest that endogenous opiates tonically inhibit the display of progesterone-facilitated lordosis in juvenile female guinea pigs.