AbstractNAD(P)H is rapidly oxidized in the presence of peroxidase, a substituted monophenol such asp‐coumaric acid, and Mn2+ions. As recently reported by Miller (1985), this NAD(P)H oxidation at the expense of molecular oxygen is inhibited by submicromolar concentrations of Cu2+ions. This inhibition by cupric ions is counteracted by micromolar concentrations of cytokinins. We now show that NAD(P)H‐oxidation by the above system is under the control of superoxide dismutase, where in the absence of cytokinin‐copper, superoxide dismutase stimulates NAD(P)H‐oxidation. In the presence of unribosylated cytokinins and copper, however, superoxide dismutase acts as an inhibitor. Thus cytokinins and superoxide dismutase may interact in the control of the redox state of plan