Sarafotoxin S6B(SRT), a peptide in snake venom, has a high degree of sequence homology with endothelin (ET) and both are potent vasoconstrictors. In order to determine whether SRT acts via the ET receptor, we performed autoradiographic binding studies on rat tissues using the radioligands, 125I-ET-1 and 125I-SRT and computerized in vitro autoradiography. In the heart, a high density of ET binding was found in the atria, and moderate density was found in the ventricles. A high density of ET-1 was found in the heart. In the kidney, ET-1 binding occurred in association with glomeruli, outer cortex, and inner stripe and inner medulla. In the adrenal, a high density of ET-1 binding occurred in the medulla as well as the zona glomerulosa. The binding affinity constant (KA) for ET-1 binding in these sites ranged from 1-10 x 10(9) M-1. Although SRT was 5-100 fold weaker than ET-1 in displacing 125I-ET-1 from these sites, 1 microM unlabelled SRT completely abolished 125I-ET-1 binding in all sites. Other venom peptides or unrelated peptides did not affect 125I-ET-1 binding. Moreover, the pattern of 125I-SRT binding in rat tissues by in vitro autoradiography was identical with that of 125I-ET-1 binding, and both unlabelled SRT and unlabelled ET-1 fully competed with 125I-SRT for binding. These results provide evidence that SRT binds to the ET binding sites in a range of rat tissues.