Because four successive weekly exposures of the gastric mucosa of intact dogs to bile did not alter the appearance or the barrier function of the mucosa during subsequent challenges with bile, the effects of chronic continuous exposure to bile were tested. This was accomplished by diversion of the flow of bile from the duodenum into the stomach by cholecystogastrostomy and diversion of the common bile duct. After four weeks, on endoscopic examination the mucosa was dark red but covered in some areas by a creamy coloured, strongly adherent pseudomembrane. Histologically the mucosa was normal. Ion fluxes, when an acid test solution was used, were close to normal. Differences between control dogs and those with chronic bile diversion became very evident, however, when the mucosa was exposed to increasing concentrations of bile. The control dogs displayed increases in net fluxes of H+, Na+, Cl- and K+ as the concentration of the bile was increased but the dogs with chronic bile diversion did not. Also the changes in PD and fluxes in K+ were less in the dogs with bile diversion. In the intact control dogs bile placed in the stomach always produced bleeding and hemorrhagic erosion; in the dogs with chronic bile diversion added bile in the stomach never caused bleeding and the mucosa appeared normal on endoscopic and histological examination. We conclude that the resistance of the gastric mucosa to the barrier breaking action of bile was increased in the dogs with chronic gastric bile diversion.