The spontaneous autoxidation of the neurotoxin 6-hydroxydopamine proceeds by a free radical chain reaction involving the superoxide anion radical and produces the corresponding chromogen 6-hydroxydopamine quinone and hydrogen peroxide. The rate of this reaction is increased in the presence of ceruloplasmin and peroxidase, and reduced by superoxide dismutase, catalase, and DT-diaphorase. We report some explanations of why these proteins may increase or reduce the rate of autoxidation of 6-hydroxydopamine.