The endothelins are a remarkable family of signaling peptides: molecular biology predicted the existence of their receptors and synthetic enzymes prior to both the identification of the encoded proteins and the synthesis of antagonists and inhibitors for use as pharmacological tools. Although considerable advances have been made, culminating in the design of endothelin antagonists with the- peutic potential in cardiovascular disease, much remains to be discovered. Tantalizingly, new research frontiers are emerging. To support further progress, Peptide Research Protocols: Endothelin encompasses experimental protocols that interrogate all facets of an endogenous mammalian peptide s- tem, from peptide and receptor expression through synthetic pathway to peptide function and potential role in human disease. Chapters describe the use of molecular techniques to quantify the expression of mRNA for both endothelin receptors and the endothelin-converting enzymes. Peptides, precursors, receptors, and synthetic enzymes may be localized and quantified in plasma, culture supernatants, tissue homogenates, and tissue s- tions using antibodies, while additional information on receptor characterization may be obtained using radioligand binding techniques. Several protocols cover in vitro assays that determine the function of the endothelin peptides in isolated preparations, that characterize new endothelin receptor ligands, or provide inf- mation on the tissue-specific processing of endothelin precursor peptides.