This article summarizes the evidence that oxy-radicals are involved in the destruction of catecholamine neurons by polyphenolic neurotoxins. Superoxide-mediated damage by 6-hydroxydopamine can be suppressed by intracellular catecholamines, which act as scavengers of the superoxide radical. Administration of exogenous hydroxyl radical-scavenging agents protects peripheral sympathetic neurons from destruction by 6-hydroxydopamine, 6- aminodopamine or 5,7-dihydroxytryptamine. Intraneuronal monoamine oxidase may drive cellular senescence of the nigrostriatal tract in patients with Parkinson's disease by generating hydrogen peroxide and derived oxy-radicals during the metabolism of endogenous dopamine.