A genetic defect in complex I of the mitochondrial electron transport chain (ETC) is implicated in the etiology of Parkinson's disease (PD), and has been studied in cybrid mitochondrial transgene cells based on the SH-SY5Y neuroblastoma. We sought to characterize further the mechanisms and time course of cell death in cultures of human SH-SY5Y neuroblastoma cells exposed to the ETC complex I inhibitor methylpyridinium ion (MPPΨ). We verify previous reports that apoptosis occurs after MPPΨ exposure in SH-SY5Y cells. Nuclear pyknosis, the end stage of apoptosis, is evident after 18-hr exposure to 5 mM MPPΨ and reversible until 10 hr, providing a temporal window within which to look for molecular and physiological correlates of MPPΨ-induced apoptosis. We then looked for mitochondrial correlates of MPPΨinduced apoptosis in SH-SY5Y cells. Using flow cytometry, we found that MPPΨ-induced increased reactive oxygen species (ROS) and lactate production consistent with inhibition of the ETC. Β°cells, lacking a functional ETC, showed no ROS production, compensatory lactate production or apoptosis after exposure to MPPΨ. Finally, we show a collapse in ROS production and mitochondrial potential that is temporally correlated with irreversibility of MPPΨ-induced apoptosis.