Alpha-synuclein is one of the main constituents of Lewy bodies and plays an important role in the pathology of Parkinson's disease. Mutation or overexpression of alpha-synuclein causes Parkinson's disease, and downregulation of alpha-synuclein resists MPP(+)-induced cell death, but the mechanism remains elusive. In this study, we attempted to explore the effect of alpha-synuclein knockdown on mitochondrial function in MPP(+)-treated SH-SY5Y cells. We reconstructed the short hairpin RNA expression vector, pGenesil-2, specially targeting alpha-synuclein mRNA, and it was stably transfected into SH-SY5Y cells. Cell viability, nuclear morphology, and mitochondrial membrane potential were then detected, and the expression of alpha-synuclein, cytochrome c, Bcl-2 and Bax were analyzed by Western blotting. The results showed that after exposure to 500 microM MPP(+) for 24 h, about 41.0+/-1.5% control cells showed low mitochondrial membrane potential. However, the percentage was 13.6+/-1.2% in MPP(+) treated alpha-synuclein knockdown cells. MPP(+) induced cytochrome c release significantly, which was about 3.1-fold compared with that of control. However, in alpha-synuclein knockdown cells, the release of cytochrome c was blocked, which was about 1.4-fold compared with that of control. The Bcl-2/Bax ratio of SH-SY5Y cells reduced to 35.5+/-3.8% after MPP(+) treatment, and this ratio was 85.2+/-3.0% in MPP(+) treated alpha-synuclein knockdown cells. These data suggest that knockdown of alpha- synuclein might be an effective means in rescuing MPP(+)-induced mitochondrial dysfunction of SH-SY5Y cells.