Peptide YY (PYY), neuropeptide Y, and pancreatic polypeptide (PP) are the third family of structurally related peptides to be extracted from the brain-gut axis. In the present study we have validated a sensitive and specific assay for PYY and used it to measure the distribution of PYY in the canine gastrointestinal tract. In addition we have compared the PYY and PP responses to both an intragastric meal and to insulin hypoglycemia. The highest concentration of PYY was present in the canine ileum (1610 +/- 123 ng/g) and colon (1607 +/- 194 ng/g). Significant (p less than 0.05) increases in both circulating PP and PYY concentrations were observed in response to the meal (mean delta PP = 180 +/- 43 pg/ml; mean delta PYY = 205 +/- 61 pg/ml). In contrast, only serum PP concentrations (mean delta PP = 294 +/- 36 pg/ml) increased significantly (p less than 0.01) in response to insulin hypoglycemia, demonstrating differences in the sensitivity of the PP and PYY cells to vagal stimulation. The present demonstration of PYY release in response to a physiological stimulus fulfills one of the prerequisites for hormonal status.