Exocrine pancreatic secretion was studied in calves as influenced by dietary protein source, collection time after feeding and age. Pancreatic duct reentrant cannulas were placed in 6 two-day-old calves fed diets containing protein from skim milk and whey powder (all-milk) or 60% from soybean flour and 40% from skim milk and whey powder (high soy). Pancreatic juice was collected before and 1, 6 and 12 hours after feeding at 4, 7, 14 and 21 days of age and assayed for protein, and for trypsin, chymotrypsin and amylase activities. Decreases in volume secretion and protein and enzyme concentration of pancreatic juice were evident at one week of age from calves fed the high soy diet and were several times less by 21 days of age, compared with calves fed the all-milk protein diet. The specific activities of trypsin and chymotrypsin were also decreased by soybean protein. Volume secretions of pancreatic juice at 4 and 21 days of age were 0.16 and 0.29 ml/hr/kg body weight, respectively. Feeding failed to evoke a significant increase in flow rate of pancreatic juice but total output of protein, trypsin and chymotrypsin tended to be greater one hour postfeeding. Activities of trypsin and chymotrypsin were highly correlated with protein content of pancreatic juice (r = 0.84 to 0.92), but were not related to volume of juice secreted. The adverse effects of the soybean protein on calf exocrine pancreatic function may be associated either with the relatively high level of soybean trypsin inhibitor in this diet, or diarrhea produced in the calves, or both.