In 21 normal fasting subjects basal serum-pancreatic-polypeptide (H.P.P.), as measured by a specific radioimmunoassay, was 43 +/- 4 pmol/l (mean +/- S.E.M). This basal concentration was of the same magnitude as that of other gastrointestinal peptide hormones. In normal subjects the H.P.P. response to food was bisphasic. A rapid eightfold increase, reaching a maximum 20--30 min after beginning of the meal, was followed by a secondary, prolonged increase. H.P.P. did not return to basal concentration within five hours. 14 duodenal-ulcer (D.U.) patients were studied before and after truncal vagotomy. Before operation the D.U. patients had higher basal H.P.P. concentrations than the normal group (110 +/- 40 pmol/l) whereas their H.P.P. response pattern to food was similar to that of the normal subjects. After vagotomy the primary rapid increase in H.P.P. concentration was eliminated and the secondary, prolonged response was significantly reduced (P less than 0-005). The results indicate that H.P.P. is a digestive hormone under vagal control.