Human endothelin-converting enzyme (ECE-1) has been shown to exist as three isoforms (ECE-1a, ECE-1b and ECE-1c) diverging in their N-terminal sequence and displaying different patterns of subcellular localization. We report here the cloning of ECE-1d, a novel isoform of 767 amino acids, which is generated from the same gene via the existence of an additional promoter located upstream from the third exon of the ECE-1 gene. ECE-1d converting activity is comparable to that of the other three isoenzymes. In contrast to ECE-1b, ECE-1d is expressed at the cell surface, although less strongly than ECE-1a. We have also shown, by identifying ECE-1b and ECE-1d in rat, that the ECE-1 diversity is conserved between human and rodent, suggesting its physiological relevance. The mRNA levels of the four isoforms were assessed in the two species in various cell types, revealing some differences. In particular, the ECE-1a isoform, strongly expressed at the plasma membrane, was found to be highly expressed in primary cultures of endothelial cells but absent from primary cultures of smooth muscle cells.