The level of superoxide radical(O^-₂) and the activities of antioxygenic enzymes in leaves were measured with plants exposed to the stress--inducing environmental factors such as low temperature, high temperature, and high intensity light. The time-courses of O₂ level change in plants which had been subjected to various environmental treatments indicated that O^-₂was indeed involved in the environmental stress. Since ocher active oxygens are produced through secondary reactions in which O^-₂takes part as a reactant, it can be inferred that the O^-₂-origenated oxygen species, such as hydroxyl radical, and hydrogen peroxide, may be also involved in the stress. The time-sequential analysis of antioxygenic enzymatic activities in the tow temperature (5℃ )-treated rice plants showed that the activities of superoxide dismutase and catalase, but not glutathione peroxidase, were markedly enhanced in accordance with the increase of O₂ level in leaves. In addition, experimental evidence that these enzymes in tissue are substrate-inducible were also obtained. From these observations, It is tempting to suggest that active oxygen species(O^-₂and O^-₂-originated species) are the major chemical intermediates, which cause damage to biological apparatuses in plant cells, and that SOD and catalase are the crucial constituents of defense system in plant cells against several environmental stresses.