Peroxidase activity was measured in spinach plants placed under various photoperiodic conditions. Continuous light (long day (LD)) always increased the peroxidase activity of leaf laminae and petioles by comparison with short days (SDs). This effect was also observed in plants having only one leaf under LD, while the other parts remained under SDs. The effect of an exogenous electrical potential on peroxide activity was tested by application of 10 V dc (current: ∼12.5 μA) during 7 days across two liquid contact electrodes on the petiole of the leaf exposed to LD. The applied potential greatly increased the effect of light on peroxidase activity, mainly on this leaf, but also on the opposite leaf which remained under SDs. Inversion of the polarity of the electrodes did not lead to an opposite effect. If the whole plant received SDs, the applied potential had only a weak effect.