To find out if and when lysosomal enzymes are excreted into pancreatic juice in physiological and pathological conditions, the changes in the secretion of cathepsin B into pancreatic juice were investigated in 66 Wistar rats with cannulation of common pancreatic-biliary duct and common bile duct, and infusions of caerulein and secretin. In a separate experiment ducts were cannulated and secretin infused as before, but in one group the ducts were "obstructed" and in another they were allowed to remain patent. Obstruction of the pancreatic duct for three hours caused a moderate significant rise in serum amylase activity. Cathepsin B activity in the pancreatic subcellular fractions was redistributed, and the amount of cathepsin B increased. In rats with obstructed ducts the secretion of cathepsin B and other lysosomal enzymes that were stimulated by caerulein was significantly greater than in the animals in which the ducts remained patent. Lysosomal enzymes associated with zymogen granules are secreted into pancreatic juice together with digestive enzymes after stimulation by gut hormones, and they may have pathophysiological roles in pancreatic juice.