Activin has been localized within cells and terminals in the brain. However, little is known about the site of action of this hormone within the brain. In the present study in vitro ligand autoradiography was used to determine the distribution of high affinity binding sites for 125I-rh activin A in rat brain. The highest density of binding sites were concentrated in the amygdala and other forebrain limbic structures. Binding was saturable in selected amygdala nuclei with an apparent Kd of approximately 0.2 nM. 125I-rh activin A binds specifically in regions known to contain terminals immunoreactive for this hormone, and also at sites distinct from the location of activin A containing neuronal terminals (e.g. the ventral limbic system).