BACKGROUND In healthy rats, combined bile and pancreatic juice diversion from gut has a synergistic rather than additive effect on stimulation of exocrine pancreatic protein secretion. We hypothesized that exclusion of combined bile and pancreatic juice from gut exacerbates bile and pancreatic-duct ligation-induced acute pancreatitis in rats to a greater extent than exclusion of either bile or pancreatic juice alone. METHODS Bile and pancreatic juice (obtained fresh from donor rats) were replaced, separately or together, via a duodenal fistula beginning immediately before 6 hours of duct ligation. Pancreatic morphologic changes were evaluated with an acute pancreatitis histology score and morphometric quantitation of acinar-cell necrosis. Plasma amylase and cholecystokinin concentrations and pancreatic subcellular distribution of cathepsin B activity were determined. Characteristics of bile and pancreatic juice obtained from donor rats were also studied. RESULTS Combined bile and pancreatic juice replacement limited the increase in acute pancreatitis histology score by 77%, acinar cell necrosis by 95%, hyperamylasemia by 77%, and hypercholecystokininemia by 99%, while preventing subcellular redistribution of cathepsin B. Amelioration of pancreatic morphologic changes was significantly greater with combined bile and pancreatic juice replacement than with replacement of either bile or pancreatic juice alone. CONCLUSIONS In this experimental corollary of early gallstone-induced acute pancreatitis, combined bile and pancreatic juice exclusion from gut contributes to disease pathogenesis to a greater extent than exclusion of either bile or pancreatic juice alone.