Autoradiographic techniques for light microscopic examination of sex steroid retention were applied to the brains of male and female Xenopus laevis, and anuran amphibian, after 3H-estradiol administration. Estrogen was concentrated by cells in three telencephalic areas (the ventral striatum, the ventral-lateral septum and the amygdala), the anterior preoptic area, the ventral thalamus, the ventral infundibular nucleus, and in the torus semicircularis. The anterior preoptic area and the ventral infundibular nucleus contained the greatest number of labelled cells. The topography of estrogen-concentrating cells was the same in male and female brains. This fact and comparisons of 3H-estradiol with 3H-testosterone retention in Xenopus suggest that the sex steroid itself, and not the genetic sex of Xenopus determines the pattern of uptake by cells in the brain. The distribution of hormone-concentrating cells in Xenopus has many similarities to that found in birds and mammals. Preoptic, hypothalamic (tuberal), limbic forebrain and specific mesencephalic sites in all these forms contain labelled cells following radioactive sex steroid administration. Findings in Xenopus add to the argument for a phylogenetically stable system of hormone-concentrating nerve cells in limbic, hypothalamic and mesencephalic structures.