Rats develop acute pancreatitis when infused iv for 3 h with cerulein (10 micrograms/kg/h). Autopsies of the pancreas seen by light microscope show interstitial edema, acinar cells vacuolization, and leukocyte margination in pancreatic capillaries; under electron microscope, severe damage concerning mitochondrial and zymogen granules structures are apparent. Particularly, swelling of the mitochondria and disruption of mitochondrial cristae was observed as well as formation of large vacuoles arising from zymogen granules and liposome fusion. A significant increase of lipid hydroperoxide level in the pancreatic tissue was observed. The purpose of this study was to evaluate the effect of 4-hydroxy-TEMPO--a low-mol-wt superoxide dismutase mimic--in a rat cerulein model of acute pancreatitis, with the expectation that free radical mediated hydroperoxide formation and tissue damage may be reduced significantly. Twenty-one male Wistar rats were divided into three groups: Group 1 (n = 5) served as a control and was infused iv for 3 h with physiologic saline; Group 2 (n = 8) was infused i.v. for 3 h with cerulein 10 micrograms/kg/h; and Group 3 (n = 8) infused i.v. both with cerulein and 4-hydroxy-TEMPO 22.6 mg/kg/h. Pancreatic tissue damage was quantified by measuring lipid hydroperoxide (LOOH) level, the weight of the organ, and by light and electron microscopic examination. 4-hydroxy-TEMPO penetration across cellular membrane barriers was quantified by ESR spectrometric measurements of 4-hydroxy-TEMPO concentration in pancreatic tissue samples and pancreatic juice as well. Administering 4-hydroxy-TEMPO to rats resulted in preventing both lipid hydroperoxide formation and severe morphological damage. 4-hydroxy-TEMPO crossed cellular membrane barriers and was excreted to pancreatic juice. Infusion of 4-hydroxy-TEMPO appears to prevent pancreatic injury caused by free radicals in experimental cerulein pancreatitis.