In an attempt to explore the mechanistic aspects of chilling injury in plants and their defensive measures against the low temperature stress, the time sequential measurements of pyruvate, superoxide radicals$(O_{overline{2}})$ and antioxygenic enzymes during whole period of injury-inducing treatment were performed using mostly rice seedlings. Pyruvate was substantialy accumulated in leaf tissues during the exposure period to $5^{circ}C$ of the seedlings ; the relative extent of the accumulation was increased with increasing time of the cold treatment. When the cold-treated plants were translocated to ambient temperature$({sim}25^{circ}C)$, the accumulation started to dissipate, concomitantly accompaning a remarkable increase in the $O_{overline{2}}$ level of tissues. Superoxide dismutase(SOD) and catalase were also activated during post-chilling period, although they showed a considerable lag time for activation. In contrast, glutathione peroxidase, another antioxygenic enzyme in cells, was not activated at all by preceding cold treatment of plants. The uptake of exogenous $O_{overline{2}}$ by the roots of rice seedlings resulted in increase in the activities of SOD and catalase in root tissues. The supply of $H_2O_2$ to plan st brought about the activation of catalase in situ, while failing to exert any effect on the activation state of glutathione peroxidase. The results obtained in this work suggest that pyruvate accumulation in cells is the direct cause of the overproduction of $O_{overline{2}}$ and thereby other toxic activated oxygen species, and that SOD and catalase may play a crucial role in the protection of plant cells against active oxygen-mediated chilling injury.