Various regions of the dog gastrointestinal tract were investigated for the distribution of peptide YY (PYY) neurons using immunocytochemistry and radioimmunoassay. PYY neurons that encircled non-PYY-immunoreactive neurons were mainly observed in the myenteric plexus from the stomach to the colon. There was more PYY-like immunoreactivity in the muscle layer of the stomach and ileum than in the other intestines. The results of high performance liquid chromatography revealed that neural PYY-immunoreactive substance is identical to authentic PYY. PYY was not localized in the cholinergic neurons. These results indicate that PYY, as a neuropeptide, is involved in the regulation of gastrointestinal function.