Cytosol and nuclear specific estradiol binding was evaluated in the fetal uterus, kidney, lung, and brain of guinea pig during fetal development and after birth. The quantity of specific binding sites increases during fetal development and decreases after birth, particularly that of the nuclei. An exception is the binding in the cytosol fraction of lung in which the number of available binding sites continues to increase in newborn animals. In the fetal uterus the amount of specific binding of estradiol and estrone is similar. The selective uptake of radioactivity and its localization by autoradiography in cell nuclei are in accordance with the high levels of estrogen receptors, particularly at the end of gestation.