Intravenous (i.v.) injection of big endothelin-1 (1-39, 0.05-1.0 nmol/kg) to anaesthetized rats produced a dose-dependent and long-lasting hypertensive effect, the magnitude of which was similar to that evoked by ET-1 (1-21). In animals given phosphoramidon (0.25 mg/kg per min i.v.), a metalloproteinase inhibitor, the hypertensive effect of big endothelin-1 was markedly attenuated. The same dose of phosphoramidon did not influence the endothelin-1-induced hypertensive effect. The possibility that big endothelin-1 is converted to endothelin-1 by a phosphoramidon-sensitive metalloproteinase in vivo warrants further attention.