Endothelin (ET)-1 is a 21-amino acid peptide with potent vasopressor and vasoconstrictive properties. Biochemical studies suggest that this peptide occurs in the adrenal cortex, where it appears to influence steroid hormone production and catecholamine release. Concomitant with our previous immunohistological study, we found ET-1 immunoreactive (IR) cells in human adrenal cortex and cortical neoplasms, but not in the medulla. These ET-1 IR cells were numerous in adenomas, but were seen only occasionally in some of the carcinomas. In the present study, the ET-1 IR protein was extracted from normal (n = 5) and hyperplastic (n=3) human cortex as well as from cortical adenomas (n = 10) and carcinomas (n = 5). Its molecular weight, determined by SDS-polyacrylamide gel electrophoresis and immunoblotting, was 9 kD, which is lower than that of prepro-ET-1 (21 kD), but larger than that of pre-ET-1 (4.3 kD) and ET-1 (2.5 kD). The normal cortical specimens, hyperplasias, adenomas and three of the five carcinomas all contained this distinct band. The two carcinomas lacking it were associated with Conn's syndrome. The protein may constitute a protein not previously described, but further studies are needed to determine its complete structure.