The cellular and subcellular localization of radioactivity in the brain of 2-day-old female rats was investigated by dry-mount and thaw-mount autoradiography 2 hr after SC injection of 1.0 µg of 2,4,6,7 3H estradiol-17β per 10 g body wt. A specific topographic pattern of nuclear concentration of estrogen was obtained similar to that of the adult rat in specific neurons of the basal hypothalamus, preoptic region and amygdala, thus providing the first in vivo evidence for estrogen receptors in the central nervous system (CNS) ofthe developing rat during the “critical period” of brain organization. In competition experiments, nuclear concentration of radioactivity was inhibited in all areas studied with estradiol, while no inhibition wasobtained with 5α-dihydrotestosterone. Testosterone clearlyinhibited the nuclear concentrationof radioactivity in all areas studied except the nucleus preopticus medialis in which either noinhibition or only a partial inhibition occurred. The implications of this selective competitionwith testosterone are discussed in relation to the binding specificity and the possible conversion of androgen to estrogen. Finally, the discrepancies between the positive autoradiographicdata and the negative biochemical data are discussed in relation to estrogen receptors and the methods used to measure these receptors. (Endocrinology94: 1386, 1974)