Serum and ascitic phospholipase A2 (PLA2) activity was assayed in rat model of acute pancreatitis. Serum PLA2 activity was increased at the onset of acute pancreatitis, but had no correlation with severity of disease. The increase of PLA2 in ascites was, on the other hand, correlated well with severity of disease. Pancreatic PLA2 activity was also increased in the course of acute pancreatitis, which reflects increase in particulate fraction and showed obviously different pattern from amylase activity. Increase of PLA2 activity on particulate fraction was due to activation of membrane bound PLA2. These data suggest that activated membrane bound PLA2 exacerbates pancreatitis. The possibility of participation of endotoxin was also to be considered.