Tissue distributions of radioactivity were studied at 2, 4 and 6 h after intravenous injection of <sup>3</sup>H-estradiol-17/J (48.6 ng/100 g b.w.) in 4 groups of 10 or 11 adult female rats with the following neonatal treatments: Control = sham ovariectomy (ovx) on day 1, oil injection on day 5 (n = 10); ovx = ovx on day 1, oil on day 5 (n = 11); TP = sham ovx on day 1, 30 μg testosterone propionate (TP) on day 5 (n = 10); ovx-TP = ovx on day 1, 30 μg TP on day 5 (n = 11). Neonatal TP treatment significantly hastened vaginal opening in all animals. After ovariectomy, vaginal opening occurred in 20/22 rats regardless of hormonal treatment; ovariectomy did not significantly affect the time of vaginal opening in these 20 animals. All ovariectomized animals had diestrous vaginal smears after puberty, all controls cycled normally and all of the TP group used for the uptake part were in persistent vaginal estrus (sequence of vaginal smears with 70% or more containing no leukocytes) confirmed by ovarian histology to be anovulatory (11/13 injected). 72 h prior to radioisotope injection, intact females in the above groups were ovariectomized and the neonatally ovariectomized were sham operated. Tissue radioactivity was extracted from the anterior, middle and posterior hypothalamus, hippocampus, amygdala, cerebrum, anterior pituitary, uterus and plasma. Neonatal ovariectomy raised <sup>3</sup>H-estradiol concentrations in all tissues, except hippocampus and anterior pituitary, suggesting an increase in nonspecifically bound hormone. Neonatal TP decreased <sup>3</sup>H-estradiol concentrations in the anterior and middle hypothalamus and uterus only, in both the intact and ovariectomized group, demonstrating a selective effect of neonatal TP upon subsequent development of estrogen binding capacities of target tissues that is independent of the ovaries.