The C-terminal α-amide structure is a characteristic feature of many biologically active peptides^1,2. Using a novel chemical method for the detection of peptide amides³, we have isolated two naturally occurring peptides, peptide HI (PHI) and peptide YY (PYY), from extracts of porcine intestine and have shown that these peptide amides represent previously unknown biologically active peptides⁴. PHI is structurally and biologically similar to vasoactive intestinal peptide (VIP)⁵ while PYY shows similarities to pancreatic polypeptide (PP)⁶. Preliminary studies indicated that PHI and PYY may both be present in brain as well as in intestine⁴. We report here the isolation from brain extracts of a peptide amide that was thought to be PYY. However, we found that the peptide, while having distinct structural and biological similarities to both PYY and PP, is a previously uncharacterized peptide, which we designate neuropeptide Y (NPY).