Abstract: Effects of osmotic stress on the production of reactive oxygen species (ROS) and change in activities of the main antioxidant enzymes, inchuding superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT) and glutathione reductase (GR), in seedlings of Secale cereale L. were investigated with 20% PEG 6000 treatment. Compared with controls, PEG-treated seedlings showed significantly increased level of membrane lipid peroxidation, contents of H2O2 and O2 -. , and activities of SOD, APX, CAT and GR. The change pattern of MDA content and ROS level in leaves was similar to that in roots, but the change in antioxidant enzyme activity induced by PEG treatment in leaves and roots differed. CAT activity in roots but not leaves of PEG-treated and control seedlings differed significantly. These results showed that SOD, APX and GR in leaves and in roots may play important roles in S. cereale in response to osmotic stress. In addition, the change in magnitude of GR activity in PEG-treated seedlings was markedly higher than that of other antioxidant enzymes, which suggests that GR may play a more important role in removing ROS than other enzymes in S. cereale.