Increasing evidence has suggested an important role for environmental toxins such as pesticides in the pathogenesis of Parkinson's disease (PD). Chronic exposure to rotenone, a common herbicide, reproduces features of Parkinsonism in rats. Mechanistically, rotenone-induced dopaminergic neurodegeneration has been associated with both its inhibition of neuronal mitochondrial complex I and the enhancement of activated microglia. Our previous studies with NADPH oxidase inhibitors, diphenylene iodonium and apocynin, suggested that NADPH oxidase-derived superoxide might be a major factor in mediating the microglia-enhanced rotenone neurotoxicity. However, because of the relatively low specificity of these inhibitors, the exact source of superoxide induced by rotenone remains to be further determined. In this study, using primary mesencephalic cultures from NADPH oxidase--null (gp91phox-/-) or wild-type (gp91phox+/+) mice, we demonstrated a critical role for microglial NADPH oxidase in mediating microglia-enhanced rotenone neurotoxicity. In neuron--glia cultures, dopaminergic neurons from gp91phox-/- mice were more resistant to rotenone neurotoxicity than those from gp91phox+/+ mice. However, in neuron-enriched cultures, the neurotoxicity of rotenone was not different between the two types of mice. More importantly, the addition of microglia prepared from gp91phox+/+ mice but not from gp91phox-/- mice to neuron-enriched cultures markedly increased rotenone-induced degeneration of dopaminergic neurons. Furthermore, apocynin attenuated rotenone neurotoxicity only in the presence of microglia from gp91phox+/+ mice. These results indicated that the greatly enhanced neurotoxicity of rotenone was attributed to the release of NADPH oxidase-derived superoxide from activated microglia. This study also suggested that microglial NADPH oxidase may be a promising target for PD treatment.