Affinity-labeling experiments with 125I-endothelin derivatives using bifunctional cross-linking reagents were carried out in an attempt to identify the polypeptide component(s) of the endothelin/sarafotoxin receptors in rat brain tissues. In rat cerebellum, cortex, and caudate putamen, endothelin 1 specifically labeled a major component with a molecular mass of around 53 000. In the same tissues endothelin 3 specifically labeled, in addition to the 53 000 band, a band of molecular mass of 38 000. This result clearly indicates that in the brain the endothelin binding site resides within a polypeptide of apparent Mr = 53 000. The possible presence of receptor subtypes is discussed with reference also to the reported identification of endothelin receptors in chick cardiac membrane and in rat mesangial cells.