In order to reproduce what may occur during the initial phase of biliary acute pancreatitis, the rabbit pancreatic duct was perfused with preincubated mixtures of bile and different digestive enzymes at low physiologic pressure. Permeability of the pancreatic duct system, serum amylase, and histological appearance of pancreatic tissue were studied after orthograde duct perfusion in the anesthetized animal. The ductal permeability was estimated by recovery of fluoresceinated dextran (molecular weight 17,200) in central venous blood following duct perfusion with this substance. Perfusion with preincubatedbile failed to increase permeability significantly (11.10 ± 3.04 nmo/L compared to 5.80 ± 2.71 nmol/L in the control group), whereas mixtures of bile and trypsin (27.19 ± 5.21 nmol/L), bile and lipase (16.68 ± 3.75 nmol/L), and bile and pancreatic juice (13.92 ± 0.48 nmol/L) caused significant increases (p < 0.05). Similar observations were made regarding serum amylase and histology. Thus, the presence of mixtures of bile with pancreatic enzymes (following their prolonged common incubation) in the absence of elevated pressure, results in an increase in duct permeability for molecules up to the size range of pancreatic enzymes and thereby may contribute to the initiation of acute pancreatitis.