Mammalian forms of neuropeptide Y (NPY) for which the amino acid sequences have previously been determined, are the human, pig, ox, rabbit, rat, and guinea-pig polypeptides. The only difference among these forms is at position 17, where pig and ox NPY have Leu and the others Met. We now show that sheep NPY differs from all the earlier characterized mammalian forms of NPY by having Asp instead of Glu at position 10. At position 17 it has Leu as do both pig and ox NPY. Consequently, 3 different structural types of mammalian NPY are now known.