Closed duodenal loop was created in the rat by obstructing the duodenum on both sides of the common bile duct. Acute haemorrhagic pancreatitis developed in these animals within 24 hours. Increased serum amylase activity, pancreatic acinar cell necrosis, pancreatic oedema and haemorrhages, and intra-abdominal fat necroses, together with formation of ascites fluid containing high levels of amylase, were observed in the experimental animals. The findings support the view that reflux of the duodenal contents into the pancreatic duct is an important early pathogenetic mechanism in the development of acute pancreatitis. The closed-loop technique in the rat offers a simple and reliable experimental model for studies on acute pancreatitis.