The promoting effect of ethanol against the cytotoxicity of hydrogen peroxide (H2O2) in differentiated PC12 cells was assessed by measuring the effect on the mitochondrial membrane permeability. Treatment of PC12 cells with H2O2 resulted in the nuclear damage, decrease in the mitochondrial transmembrane potential, cytosolic accumulation of cytochrome c, activation of caspase-3, increase in the formation of reactive oxygen species (ROS) and depletion of GSH. In PC12 cells and dopaminergic neuroblastoma SH-SY5Y cells, the promoting effect of ethanol on the H2O2-induced cell death was increased with exposure time. Ethanol promoted the nuclear damage, change in the mitochondrial membrane permeability, ROS formation and decrease in GSH contents due to H2O2 in PC12 cells. Catalase, carboxy-PTIO, Mn-TBAP, N-acetylcysteine, cyclosporin A and trifluoperazine inhibited the H2O2 and ethanol-induced mitochondrial dysfunction and cell injury. The results show that the ethanol treatment promotes the cytotoxicity of H2O2 against PC12 cells. Ethanol may enhance the H2O2-induced viability loss in PC12 cells by promoting the mitochondrial membrane permeability change, release of cytochrome c and subsequent activation of caspase-3, which is associated with the increased formation of ROS and depletion of GSH. The findings suggest that ethanol as a promoting agent for the formation of mitochondrial permeability transition may enhance the neuronal cell injury caused by oxidants.