Cisplatin-induced nephrotoxicity was studied in porcine proximal tubular cells, focusing on the relationship between mitochondrial damage, reactive oxygen species (ROS) and cell death. Cisplatin specifically affected mitochondrial functions: complexes I to IV of the respiratory chain were inhibited 15 to 55% after 20 min of incubation with 50 to 500 microM, respectively. As a result, intracellular ATP was decreased to 70%. The mitochondrial glutathione (reduced form) (GSH)-regenerating enzyme GSH-reductase (GSH-Rd) activity was reduced by 20%, which contributed to a 70% reduction of GSH levels and ROS formation. The residual electron flow through the mitochondrial respiratory chain was the source of ROS because additional inhibition of the complexes I to IV reduced ROS formation. Because cisplatin affects both GSH-Rd and complexes I to IV, cells were incubated with N,N'-bis(2-chloroethyl)-N-nitrosourea (inhibitor of GSH-Rd) and inhibitors of the different complexes. Only N,N'-bis(2-chloroethyl)-N-nitrosourea with rotenone (complex I inhibitor) induced ROS formation, which indicates that inhibition of complex I and inhibition of the GSH-Rd is probably the cause of ROS formation. However, the resulting ROS is not the cause of cell death because diphenyl-p-phenylene-diamine and deferoxamine, which completely prevented ROS, could not prevent cell death. Similarly, the antioxidants did not completely prevent the decrease in activity of complexes I to IV, ATP or GSH levels. In conclusion, ROS formation does occur during cisplatin-induced toxicity, but it is not the direct cause of cell death.