The purification of NPY from brains of the American alligator (Alligator mississippiensis) was achieved using reverse-phase high performance liquid chromatography (HPLC). The amino acid sequence was determined using automated Edman degradation as Tyr-Pro-Ser-Lys-Pro-Asp-Asn-Pro-Gly-Glu- Asp-Ala-Pro-Ala-Glu-Asp-Met-Ala-Arg-Tyr-Tyr-Ser-Ala-Leu-Arg-His-Tyr-Ile- Asn-Leu - Ile-Thr-Arg-Gln-Arg-Tyr. Alligator NPY is the first non-mammalian vertebrate to have 100% sequence identity to human NPY. The conservation of alligator NPY suggests that serine in position 7 of chicken NPY evolved after the birds and reptiles diverged from a common Archosaurian ancestor. Furthermore, the sequence identity between alligator and human NPY suggests this sequence is the same as the ancestral amniote NPY.