We studied morphologic changes in a rat model of acute hemorrhagic pancreatitis in order to investigate the mechanism by which water immersion stress injures the pancreas. Acute hemorrhagic pancreatitis was induced by two intraperitoneal injections of 40 micrograms/kg body weight of caerulein at 1-h intervals under water immersion stress for 5 h at 23 degrees C. Light microscopy showed interstitial edema with inflammatory cell infiltration, degeneration and necrosis of acinar cells, and bleeding. Electron microscopy showed large autophagic vacuoles, decreased zymogen granules, and dilated rough endoplasmic reticulum in acinar cells. Basolateral exocytosis of large vacuoles and phagocytosis of the degenerated acinar cells were observed. In addition, microvascular damage, including the destruction of the capillary endothelial cells, capillary thrombosis, and the extravasation of blood cells, was seen. In contrast, in a pancreatitis model induced by caerulein injection alone, there was no bleeding, no remarkable vascular change, and no thrombosis. Degeneration and necrosis of acinar cells were less severe. In the pancreas under stress alone, microvascular damage and degeneration of acinar cells were observed. These findings demonstrate that stress injures the pancreas and worsens the pancreatitis by causing microcirculatory disturbances, such as vascular damage, thrombosis, increased vascular permeability, and bleeding. These results suggest that chemical mediators, such as free radicals and platelet-activating factor (PAF), which are produced by vascular damage and thrombosis, may accelerate the activation of zymogen proteases in acinar cells in caerulein-induced pancreatitis, leading to hemorrhagic pancreatitis.