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The present study examined the combined effect of dopamine and 1-methyl-4-phenylpyridinium (MPP+) on the membrane permeability in isolated brain mitochondria and on cell viability in PC12 cells. MPP+ increased effect of dopamine against the swelling, membrane potential, and Ca2+ transport in isolated mitochondria, which was not inhibited by the addition of antioxidant enzymes (SOD and catalase). Dopamine or MPP+ caused the decrease in transmembrane potential, increase in reactive oxygen species, depletion of GSH, and cell death in PC12 cells. Antioxidant enzymes reduced each effect of dopamine and MPP+ against PC12 cells. Co-addition of dopamine and MPP+ caused the decrease in the transmembrane potential and increase in the formation of reactive oxygen species in PC12 cells, in which they showed an additive effect. Dopamine plus MPP+-induced the depletion of GSH and cell death in PC12 cells were not decreased by the addition of antioxidant enzymes, rutin, diethylstilbestrol, and ascorbate. Melanin caused a cell viability loss in PC12 cells. The N-acetylcysteine, N-phenylthiourea, and 5-hydroxyindole decreased the cell death and the formation of dopamine quinone and melanin induced by co-addition of dopamine and MPP+, whereas deprenyl and chlorgyline did not show an inhibitory effect. The results suggest that co-addition of dopamine and MPP+ shows an enhancing effect on the change in mitochondrial membrane permeability and cell death, which may be accomplished by toxic quinone and melanin derived from the MPP+-stimulated dopamine oxidation.