Developmental changes in the positive feedback effects of ovarian steroids on LH release in female guinea pigs were studied. Administration of estradiol benzoate (EB; 10 micrograms) to intact females on day 30 of age (prepubertal) elicited a LH surge in only 1 of 14 instances. However, by day 46 of age (peripubertal), EB induced a sharp rise in serum LH concentrations in 9 of 12 females. The magnitude and timing of the LH surge in these peripubertal animals were similar to those in cycling adults given EB on day 13 of the estrous cycle. Progesterone (P; 1.0 mg) administration to intact females 30 h after a priming dose (1.5 microgram) of EB was also effective in eliciting a LH surge on day 46, but not on day 30, of age. The magnitude of the P-induced LH surge was significantly greater (P less than 0.001) than that of the EB-induced LH surge on day 46 of age. Pentobarbital anesthesia delayed, but did not prevent, the EB-induced LH surge in peripubertal females, while it completely abolished the P-induced LH surge. These results suggest that ovarian steroids can stimulate LH release in the immature female guinea pig, but not until an age approaching the normal time of the first ovulation. Those observations together with results presented in a preceding paper suggest that establishment of the positive feedback system between days 30 and 46 of age is associated with an increased capacity of the hypothalamic-pituitary axis to stimulate LH release; such developmental changes may represent the major events leading to the onset of puberty in the female guinea pig.