Hepatotoxicity of diclofenac has been known in experimental animals and humans but its mechanism has not been fully understood. The present study examined the role of mitochondrial permeability transition (MPT) in the pathogenesis of diclofenac-induced hepatocyte injury by using isolated mitochondria and primary culture hepatocytes from rats. Incubation of energized mitochondria with succinate in the presence of Ca2+ and diclofenac resulted in mitochondrial swelling, leakage of accumulated Ca2+, membrane depolarization, and oxidation of nicotinamide adenine dinucleotide phosphate and protein thiol. All of these phenomena were suppressed by coincubation of the mitochondria with cyclosporin A, a typical inhibitor of MPT, showing that diclofenac opened the MPT pore. It was also suggested that reactive oxygen species probably generated during mitochondrial respiration and/or voltage-dependent mechanism was involved in MPT, which are proposed as mechanisms of MPT by uncouplers of mitochondrial oxidative phosphorylation. Culture of hepatocytes for 24 hours with diclofenac caused a decrease in cellular ATP, leakage of lactate dehydrogenase and membrane depolarization. The hepatocyte toxicity thus observed was attenuated by coincubation of the hepatocytes with cyclosporin A and verapamil, a Ca2+ channel blocker. In conclusion, these results showed the important role of MPT in pathogenesis of hepatocyte injury induced by diclofenac and its possible contribution to human idiosyncratic hepatotoxicity.