Penetration of lanthanum through the main pancreatic duct epithelium in cats following exposure to infected human bile. The main pancreatic duct epithelium acts as a barrier to the diffusion of molecules from the duct lumen into pancreatic acinar and interstitial tissue. We studied sequential ultrastructural characteristics of the loss of epithelial barrier function in the cat using lanthanum, an electron-opaque tracer, following perfusion of the duct from the tail to the duodenum with infected human bile. Tight junctions between duct epithelial cells were found to become permeable to the tracer as early as after 15 min of exposure. Later, there was progressive disintegration of intercellular junctions and epithelial loss. Lanthanum penetrated the duct epithelium exclusively on an intercellular path. Loss of barrier function of the pancreatic duct epithelium was consistently associated with subsequent development of acute interstitial edematous pancreatitis. There was no association between the degree of duct epithelial damage and the severity of acute pancreatitis. Both bile and a suspension of bacteria alone were not harmful to the pancreas. Sequential perfusion produced acute pancreatitis only when at first bile and then the bacterial suspension was perfused. A reversed succession of perfusates produced no morphologic alterations. We conclude: (1) Increased tight junction permeability is an early lesion in acute bile-induced pancreatitis: (2) loss of duct epithelial barrier function is important for the initiation but not for the severity of the inflammation; and (3) bile renders duct epithelial intercellular junctions vulnerable to Escherichia coli bacteria. 