The effect of atropine on exocrine pancreatic secretion was investigated in conscious rats. Intravenous atropine infusion decreased nonstimulated protein secretion during recirculation of pancreatic juice into the duodenum D50 = 15-20 micrograms/kg/h. The maximum inhibition from protein secretion (-89%) was obtained with 600 micrograms/kg/h. With larger doses, the inhibition was less. The response to secretin and cholecystokinin-pancreozymin was not significantly modified by atropine. When pancreatic juice was diverted during the course of an intravenous atropine infusion, the first 1-hour peak of protein output was significantly decreased, but the following 2-hour period was increased, the sum of these 2 periods being similar in both conditions. The response to soybean trypsin inhibitor during recirculation was decreased as well as the first peak after diversion. During atropine infusion fluid secretion decreased more powerfully after 1 h diversion and after soybean trypsin inhibitor than during recirculation of pancreatic juice. It is suggested that during recirculation of pancreatic juice nonstimulated protein secretion is mostly (89%), and water secretion is partially controlled by cholinergic mechanisms. After soybean trypsin inhibitor stimulus and during the early phase following juice diversion protein secretion seems to be partly under the control of cholinergic mechanisms. However, during the latter phase following diversion, it is not so. Parasympathetic stimulation appears also to play a significant, although less important, role in fluid secretion.