The significance of pyloric reflux of bile and duodenal juice in experimental gastric stress ulceration was studied using a swine shock ulcer model. Following a haemorrhagic shock of 3 h duration, nine of the ten control piglets (90%) with normal bile flow have gastric mucosal lesions. None of the five piglets with permanent bile diversion (ligation of common bile duct; cholecystojejunostomy) has gastric lesions. Intragastric instillation of pure bile prior to the shock in piglets with permanent bile diversion induced gastric lesions in only one of the five test animals (20%). If duodenal juice was used instead, four of the six test animals (67%) have lesions. The results indicate that, in the pig, an uninterrupted flow of bile into the duodenum is a prerequisite for the development of gastric mucosal lesions following haemorrhagic shock. They also suggest that duodenal juice containing both bile and pancreatic juice has a stronger ulcerogenic influence on shocked porcine gastric mucosa than bile alone.