In dogs with gastric and pancreatic fistulas, we studied the effect of intravenous infusion of atropine in doses of 0.9, 1.8, 7, and 29 nmol X kg-1 X h-1 on the pancreatic secretory response to graded loads of intraduodenal infusions of tryptophan, given with a secretin background. Infusions of 1.8, 7, and 29, but not 0.9 nmol X kg-1 X h-1 of atropine sulfate significantly (P less than 0.05) decreased the incremental protein response to all loads of tryptophan. The cumulative incremental protein output was reduced by 44, 37, and 52%, respectively. Infusions of 1.8, 7, and 29 nmol X kg-1 X h-1 of atropine significantly decreased by approximately 50% the incremental bicarbonate response to low (0.12 and 0.37 mmol/h) but not high loads (1.1, 3.3, and 10 mmol/h) of tryptophan. The inhibitory potency of the effective doses of atropine did not differ significantly. Only the highest dose of atropine significantly increased heart rate by 76%. These findings indicate that 1) in the intact animal, the minimal dose of atropine required for inhibition of pancreatic bicarbonate and protein response to intraduodenal tryptophan seems to be 1.8 nmol X kg-1 X h-1, a dose that causes probably few systemic effects, since it does not increase heart rate; and 2) the inhibitory action of atropine on the pancreatic response to tryptophan appears to be an "all-or-none" effect.