Studies have been performed on pure pancreatic juice obtained by direct cannulation of the pancreatic duct in 2 patients with acute pancreatitis. The striking abnormalities observed, which were in marked contrast to our observations in 15 normal subjects, were high concentrations of protein throughout the period of secretin stimulation and the sporadic appearance of free proteolytic activity in many 1-min specimens throughout the collection period. In 1 subject repeat studies were performed after resolution of the pancreatitis when the profile observed was normal. Our findings are consistent with the hypothesis that obstruction of ductules and intraductal activation of zymogens may be important in the pathogenesis of acute pancreatitis.