Atrial natriuretic peptide (ANP), a hormone secreted from mammalian atria, regulates the homoeostatic balance of body fluid and blood pressure. ANP-like immunoreactivity is also present in the brain, suggesting that the peptide functions as a neuropeptide. We report here identification in porcine brain of a novel peptide of 26 amino-acid residues, eliciting a pharmacological spectrum very similar to that of ANP, such as natriuretic-diuretic, hypotensive and chick rectum relaxant activities. The complete amino-acid sequence determined for the peptide is remarkably similar to but definitely distinct from the known sequence of ANP, indicating that the genes for the two are distinct. Thus, we have designated the peptide 'brain natriuretic peptide' (BNP). The occurrence of BNP with ANP in mammalian brain suggests the possibility that the physiological functions so far thought to be mediated by ANP may be regulated through a dual mechanism involving both ANP and BNP.