To perform nondestructive real-time monitoring of the redox state in plants against environmental stress, a 700-MHz surface-coil-type ESR resonator was employed and temporal changes in ESR signals of an exogenous nitroxide radical in the leaves of a potted tobacco plant were observed after chilling. ESR signals of the nitroxide radical were barely observed at room temperature, indicating that the nitroxide radical was reduced to the hydroxylamine by reductants in the leaves. Chilling significantly increased die signal-to-noise ratios (S/Ns) of the nitroxide radical in the leaves. Because hydroxylamines are oxidized to nitroxide radicals by oxidants, it is thought that the ESR S/Ns can be used as an index for oxidation in leaves.