At neutral pH, 1,4-naphthohydroquinone and 2-methyl-1,4-naphthohydroquinone readily autoxidize to the corresponding quinones. In an unpurified phosphate buffer, the autoxidation of both substances proceeded in a linear fashion after a brief lag phase. Addition of a chelating agent or purification of the buffer decreased the duration of the lag phase of 1,4-naphthohydroquinone autoxidation, but had no effect on the linear rate. In the case of 2-methyl-l,4-naphthohydroquinone, such treatment eliminated the lag phase and greatly increased the linear rate of oxidation. The lag phases and oxidation rates seen in unpurified buffers could be replicated by addition of submicromolar amounts of copper to purified buffer. The effects of low levels of copper were qualitatively similar to those of superoxide dismutase, and it is suggested that the effects of this metal on naphthohydroquinone autoxidation reflects its ability to act as a superoxide dismutase. The relative rates of autoxidation of naphthohydroquinones are important because they may determine the balance between activation and detoxication of naphthoquinones within cells. When measuring such rates, or assessing rates of redox cycling of naphthoquinones, it is important to employ a chelating agent or use highly purified buffers and reagents. Failure to do so may lead to erroneous conclusions concerning the reactivity of naphthohydroquinones and the ability of naphthoquinones to generate "active oxygen" species.