A growing body of evidence indicates that an inflammatory process in the substantia nigra, characterized by activation of resident microglia, likely either initiates or aggravates nigral neurodegeneration in Parkinson's disease (PD). To study the mechanisms by which nigral microglia are activated in PD, the potential role of alpha-synuclein (a major component of Lewy bodies that can cause neurodegeneration when aggregated) in microglial activation was investigated. The results demonstrated that in a primary mesencephalic neuron-glia culture system, extracellular aggregated human alpha-synuclein indeed activated microglia; microglial activation enhanced dopaminergic neurodegeneration induced by aggregated alpha-synuclein. Furthermore, microglial enhancement of alpha-synuclein-mediated neurotoxicity depended on phagocytosis of alpha-synuclein and activation of NADPH oxidase with production of reactive oxygen species. These results suggest that nigral neuronal damage, regardless of etiology, may release aggregated alpha-synuclein into substantia nigra, which activates microglia with production of proinflammatory mediators, thereby leading to persistent and progressive nigral neurodegeneration in PD. Finally, NADPH oxidase could be an ideal target for potential pharmaceutical intervention, given that it plays a critical role in alpha-synuclein-mediated microglial activation and associated neurotoxicity.