Human chymase converts big endothelin (ET)-1 to 31-amino acid length ET-1 {ET-1(1-31)} that also possesses a potent vasoconstrictive action. In addition, ET-1(1-31) is an intermediate peptide, which is then readily transformed to mature ET-1 by the neutral endopeptidase 24-11. To investigate the relevance of pathophysiology of ET-1(1-31) in vivo, we have developed specific sandwich-type, enzyme-linked immunosorbent assay to measure the plasma concentration of ET-1(1-31) in healthy volunteers and patients with myocardial infarction. The plasma concentrations of ET-1(1-31) in healthy volunteers were 24.8 ± 5.2 pg/ml (n=11). ET-1(1-31) concentration in plasma was elevated in patients with acute myocardial infarction, and its elevation was several times higher and lasted longer than that of ET-1. In addition, tissue concentration of ET-1(1-31) in the myocardium from a patient with acute myocardial infarction was extremely high (12729.8 ± 2617.7 pg/mg protein). These results suggest that ET-1(1-31) may play some pathological roles in the remodeling, especially in sites where inflammatory cells produced a large amount of proteases, such as myocardial infarction.