The autoxidation of 6-hydroxydopamine (6HODA) was virtually blocked (k2 less than 10(-15) M-1 S-1 at pH 8.0, ionic strength 0.04) by the simultaneous presence of diethylenetriaminepentaacetic acid (DTPA), catalase, and superoxide dismutase (SOD). No quinone product or oxygen consumption was detectable after 12 min under these conditions. Thus, if 6HODA is to react with molecular oxygen at a measurable rate, some other redox species is required as a coreductant. The subsequent addition of formate or mannitol proved capable of overcoming the total inhibition induced by the mixture of catalase, SOD, and DTPA. The simplest interpretation of the data is that most of the autoxidation of 6HODA, as commonly observed, involves successive reduction of a series of metal-bound species of oxygen; the actual transfer of electrons occurring within a ternary reductant-metal-oxygen transition state.