A single i.p. dose of 1 µC of 1,2<sup>3</sup>H-testosterone (specific activity 147 µCi/ µg) and 2 µC of 17 β,6,7,-3H-estradiol (specific activity 178 µCi/ µg) was injected into 2- to 10-day-old female rats and its distribution was studied after 5, 15, and 60 min in blood, cerebral cortex, hypothalamus, pituitary gland, liver, uterus, and spleen. The hypothalamus did not show preferential uptake of testosterone at the times studied. The amount of radioactivity per unit weight in the preoptic and ventral hypothalamus due to the testosterone injected was similar to the amount per unit weight found in the cerebral cortex at all times studied. Radioactivity levels recorded in the liver of the rats injected with testosterone suggest a metabolic activity toward this steroid, which is in marked contrast with the moderate uptake of the tritiated estradiol by hepatic tissue. Estrogenic and androgenic radioactivity distribution in the spleen was similar to that of the cerebral cortex control value. Active uptake of estrogen by the pituitary gland of these animals was not observed under the experimental conditions used. This fact cannot be explained by an incomplete maturation of the portal vessel system, because testosterone diffuses easily into the gland. However, when anterior pituitary lobes were incubated in vitro with the tritiated estrogen, a typical active uptake curve was observed, suggesting that the pituitary estradiol receptors are somehow masked by the physiological condition of the neonatal rat. In contrast to the pituitary, the uterus actively takes up labeled estrogen, in vivo, while testosterone is not accumulated but quickly diffuses out of the organ. Thin-layer chromatography studies showed that estrogen and testosterone are, apparently, somehow rapidly metabolized into more polar derivatives in the neonatal rat.