Autoradiographic methods were used to study the location of estrogen-concentrating cells in the brain of the female hamster. In the hypothalamus, well-labelled cells were reliably found in the posterior medial preoptic area (MPOA), the anterior hypothalamus (AHA), and the ventromedial (VM), arcuate (ARC) and ventral premammillary nuclei (VPM). In the limbic system, well-labelled cells were found in the ventro-lateral septum, bed nucleus of the stria terminalis, and the medial and cortical nuclei of the amygdala. Labelled cells, in small numbers, were also detected in the mesencephalic central gray (CG), lateral hypothalamus, subiculum and entorhinal cortex. The neuro-anatomical pattern of estrogen-concentrating cells in the hamster supports the concept of a generalized vertebrate pattern. Furthermore, a comparison of hamster and rat patterns of cellular <sup>3</sup>H-estradiol (<sup>3</sup>H-E<sub>2</sub>) concentration appears to suggest that species differences in their responsiveness to estrogen may be paralleled by differences in estrogen binding.