Brain lesions containing filamentous and aggregated α-synuclein are hallmarks of neurodegenerative synucleinopathies. Oxidative stress has been implicated in the formation of these lesions. Using HEK 293 cells stably transfected with wild-type and mutant α-synuclein, we demonstrated that intracellular generation of nitrating agents results in the formation of α-synuclein aggregates. Cells were exposed simultaneously to nitric oxide- and superoxide-generating compounds, and the intracellular formation of peroxynitrite was demonstrated by monitoring the oxidation of dihydrorhodamine 123 and the nitration of α-synuclein. Light microscopy using antibodies against α-synuclein and electron microscopy revealed the presence of perinuclear aggregates under conditions in which peroxynitrite was generated but not when cells were exposed to nitric oxide- or superoxide-generating compounds separately. α-Synuclein aggregates were observed in 20–30% of cells expressing wild-type or A53T mutant α-synuclein and in 5% of cells expressing A30P mutant α-synuclein. No evidence of synuclein aggregation was observed in untransfected cells or cells expressing β-synuclein. In contrast, selective inhibition of the proteasome resulted in the formation of aggregates detected with antibodies to ubiquitin in the majority of the untransfected cells and cells expressing α-synuclein. However, α-synuclein did not colocalize with these aggregates, indicating that inhibition of the proteasome does not promote α-synuclein aggregation. In addition, proteasome inhibition did not alter the steady-state levels of α-synuclein, but addition of the lysosomotropic agent ammonium chloride significantly increased the amount of α-synuclein, indicating that lysosomes are involved in degradation of α-synuclein. Our data indicate that nitrative and oxidative insult may initiate pathogenesis of α-synuclein aggregates.