A peptide exhibiting natriuretic and vasodepressor activity was isolated from eel brains. Its amino acid sequence was found to be similar to, but distinct from, that of the eel atrial natriuretic peptide (ANP), and it is characterized by the absence of the C-terminal sequence that follows the second-half cystine. The extent of sequence homology of this peptide to known mammalian brain natriuretic peptides (BNPs) is greater than to ANPs. Therefore, we have named this peptide "eel BNP-like peptide". The isolation of this peptide provides the first evidence for the presence of two different molecular types of natriuretic peptide in a single non-mammalian species. The potency of the eel BNP-like peptide relative to that of human ANP, in terms of its vasodepressor activity, was 117 in the eel, 1.7 in the quail, and 0.08 in the rat. Thus, eels exhibit a high degree of sensitivity to this native peptide just as they do to eel ANP.