Endothelin-1 (ET-1), the first member of the newly discovered mammalian endothelin family of biologically active peptides, was originally identified as a 21 residue potent vasoconstrictor peptide in vascular endothelial cells. However, it has since been demonstrated to possess a wide variety of pharmacological activities in tissues both within and outside the cardiovascular system, and peptides with a striking similarity to ET-1 have been found to be the major toxic component of a snake venom. Moreover, recent studies have suggested that mammals including humans produce three distinct members of this peptide family, ET-1, ET-2 and ET-3, which may have different profiles of biological activity, and may act on distinct subtypes of endothelin receptor. Masashi Yanagisawa and Tomoh Masaki review the current status of the biochemistry and molecular biology of endothelin.