Abstract:The seedlings of hybrid rice were exposed to low(l℃) and high (40℃) temperatures and the permeability of plasma membrane, the activities of SOD, catalase, and peroxidase were assayed and the contents of malondialdehyde (MDA), ascorbic acid and glutathione were estimated. The ability of the seedlings to scavenge activated oxygen progresively decreased with the time of exposure to low temperature, concurrently MDA gradually accumulated and the permeability of the plasma membrane decreased. The amounts of MDA increased were different in the two hybrid combinations and were not in accordance with the ability of cold resistance. Comparing the more resistant variety “Wei-you 287” with the less resistant variety “Wei-you 49”, it was found that the leakage of electrolytes, the activities of SOD, peroxidase and catalase and the content of glutathione were all higher in the more resistant variety, but the accumulation of MDA was more in the less resistant variety. Higher temperature was less harmful than lower one to the seedlings of hybrid rice. Both enzymic and non-enzymic systems scavenging activated oxygen were similarly important in protecting the seedlings from damage by such oxygen.