Paraquat (PQ) is a cationic non-selective bipyridyl herbicide widely used in agriculture to control weeds and grasses. Epidemiologic studies indicate that exposure to pesticides can be a risk factor in the incidence of Parkinson`s disease (PD). A strong correlation has been reported between exposure to paraquat and PD incidence in Canada, Taiwan, and United States. This correlation is supported by animal studies showing that paraquat produces toxicity in dopaminergic neurons of the rat and mouse brain. However, it is unclear how paraquat triggers toxicity in dopaminergic neurons. Based on the previous reports, it was hypothesized that paraquat may induce oxidative stress and proteasomal dysfunction-mediated toxicity in dopaminergic neurons. To explore this possibility, dopaminergic SH-SY5Y human neuroblastoma cells were treated with paraquat, and several biomarkers of oxidative stress or proteasomal dysfunction were investigated. First, a specific dopamine transporter inhibitor GBR12909 significantly protected SY5Y cells against the toxicity of paraquat, indicating that paraquat exerts its toxicity by a mechanism involving the dopamine transporter (DAT). Second, paraquat increased the levels of reactive oxygen species (ROS) in SY5Y cells, but decreased the levels of glutathione. Third, paraquat inhibited glutathione peroxidase activity, but did not affect glutathione reductase activity. On the other hand, paraquat increased GST activity by 24 hr, after which GST activity returned to the control value at 48 hr. Fourth, paraquat decreased mitochondrial transmembrane potential (MTP). Fifth, paraquat produced the increases in malondialdehyde (MDA) and protein carbonyls, as well as DNA fragmentation, indicating oxidative damage to major cellular components. Sixth, paraquat decreased proteasomal activity, the activities of mitochondrial complex I and V, and intracellular ATP levels, but increased the activities of caspase 3 and 9, indicating that proteasomal inhibition is linked to mitochondrial dysfunction accompanied by the activation of apoptotic signaling pathway. Seventh, paraquat increased the protein levels of heme oxygenase-1 (HO-1), p53, Bax, α-synuclein and ubiquitinated proteins. Eighth, paraquat induced nuclear condensation. Taken together, these findings support the hypothesis that paraquat produces oxidative stress and proteasomal dysfunctionmediated toxicity in SY5Y cells. Thus, current findings suggest that paraquat may induce the pathogenesis of dopaminergic neurons through oxidative stress and proteasomal dysfunction.