In dogs with gastric and pancreatic fistulas, we studied the effect of atropine on the pancreatic secretory response to secretin and intestinal HCl. Atropine sulfate (20 micrograms.kg-1.h-1 iv) significantly depressed basal bicarbonate and protein output. Atropine depressed bicarbonate responses to low doses (62.5, 125, 250, and 500 ng.kg-1.h-1) of secretin but had no significant effect on responses to high doses (1,000 and 2,000 ng.kg-1.h-1). Secretin, with or without atropine, did not stimulate pancreatic protein output above basal. Atropine depressed bicarbonate responses to low loads (3, 6, and 12 mmol.h-1) of HCl but had no significant effect on responses to high loads (12, 24, and 48 mmol.h-1). Intraduodenal HCl produced a dose-dependent increase in protein output. Atropine abolished protein responses to low loads (3 and 6 mmol.h-1) but did not affect responses to high loads (24 and 48 mmol.h-1) of HCl. These findings are compatible with the hypotheses that a) endogenous cholinergic activity augments the pancreatic bicarbonate response to secretin, and b) the pancreatic protein response to intraduodenal HCl is, at least in part, mediated cholinergically.