We have characterized the endothelin-converting enzyme (ECE)-like activity involved in big endothelin (ET)-1-induced contraction in rabbit saphenous artery (RSA). Big ET-1 30 nM caused a contraction that was independent of the vascular endothelium. Phosphoramidon and the neutral endopeptidase (NEP) inhibitors thiorphan and candoxatrilat blocked the vasoconstriction caused by big ET-1 in endothelium-denuded RSA. Candoxatrilat (IC50 17 nM) and thiorphan (IC50 2.5 nM), were 5- to 30-fold more potent than phosphoramidon (IC50 83 nM). Other protease inhibitors were inactive. In cultured endothelial cells the ET-1 release was inhibited only by phosphoramidon (IC50 16 microM) but at a concentration 200-fold that required an endothelium-denuded RSA. In conclusion, we can speculate that the big ET-1 contraction in RSA is mediated by an ECE, probably present on smooth muscle cells, which is susceptible to NEP inhibitors and is different from the ECE on endothelial cells.