The effect of cholinergic blockade on the concentrations of pancreatic-polypeptide (PP) in plasma was studied in 19 patients with duodenal ulcer (DU) and 16 control subjects. PP concentrations increased with age both in control subjects and in DU patients. In the DU patients atropine or benzilonium, an antimuscarinic agent with minimal cerebral actions, reduced PP concentrations from 47 (8-220) to 28 (7-53) pmol/liter, n = 18, median and total range. In the control subjects atropine suppressed the PP concentrations from 17 (0-257) to 11 (0-41) pmol/liter, n = 15. Cholinergic blockade had only a minor effect in 1 patient and 1 control, both with high PP concentrations. Both in the DU patients and in the control group the suppression by the cholinergic blockade was most marked in subjects with elevated PP concentrations. Spontaneous acid and PP secretion were measured simultaneously in 25 DU patients. No correlation was found between median acid secretion and median concentrations of PP. However, in the individual patient a positive covariation was found between fluctuations in spontaneous acid secretion and fluctuations in PP concentration. P less than 0.005. We conclude that plasma concentrations of PP in the basal state are suppressible by cholinergic blockade and that PP concentrations fluctuate synchronously with the spontaneous secretion of gastric acid. These results suggest that PP concentrations in plasma before and after cholinergic blockade may possibly serve as indicator of abdominal vagal tone.