In experiments with dogs the influence of controlled respiration with positive endexpiratory pressure (PEEP) on the pancreas was investigated. The pO2 within the tissue was measured during the time of respiration. At PEEP 10 and PEEP 20 an average diminution was observed in the tissue pO2 of 27% and 37%, respectively. A pancreatic edema produced after PEEP 20 was changing into a necrotizing pancreatitis during the following 24 h. At PEEP 10, such a transition was not observed. The pancreatic edema was accompanied by the typical increase in alpha-amylase and lipase activities. After 24 h there were small changes of the enzyme activities in the serum at PEEP 10, whereas at PEEP 20 they were remarkably increased. These results demonstrate that PEEP 20 causes a shortage of oxygen supply of the pancreas. This shortage in connection with an edema can provoke an acute pancreatitis.