In the male rat, a dramatic increase in hypothalamic testosterone and estradiol concentrations occurs during the first few hours of postnatal life. These experiments sought to determine whether such increases participate in the defeminization of positive estrogen feedback effects on LH secretion. Newborn male rats were castrated either in utero (0 h males), or 10 or 24 h after birth. Some males were castrated at 0 h in utero and injected at the time of surgery with 1,2.5, or 5 micrograms testosterone propionate. A group of females was ovariectomized at 0 h in utero (0 h females). The control group consisted of male and female rats sham gonadectomized at 0 h in utero which were either gonadectomized at 21 days of age or left intact. The experimental groups were challenged before puberty to determine if estrogen induced a release of LH using two different types of estrogen treatment. The first treatment consisted of an injection of 0.2 microgram estradiol benzoate (EB) on day 28 followed by a second 10 micrograms injection of EB on day 29. This treatment resulted on the afternoon of day 30 in a surge of LH in intact females. Normal males, 0 h males, or females castrated at 21 days did not have a significant LH surge. The second test consisted of the daily injection of 0.05 microgram EB on days 23-27; on day 28 the rats were injected with 2.5 micrograms EB. Zero hour male and female rats showed a large LH surge on the afternoon of day 29; sham castrated males never responded to this treatment. No sex difference was observed in the mean size of the LH surge providing the males were castrated at 0 h in utero. The effect of the hour of castration on the day of birth also was studied. Males castrated at 10 or 24 h after birth showed either no LH surge or the magnitude of the surge was greatly reduced compared to that obtained in the 0 h males (P less than 0.001). The fact that 0 h males injected with 1 microgram testosterone propionate never showed an LH surge after prepuberal treatment with estrogen suggests that 0 h is a time during which the newborn is sensitive to the defeminizing effect of androgens. These results are consistent with the idea that the testicular hyperactivity which occurs at the time of birth could influence the defeminization of the LH surge mechanisms.