Abstract: Effects of the exogenous NO donor sodium nitroprusside (SNP) on the ascorbate-glutathione cycle dependent antioxidant enzyme activities, contents of antioxidant substance, H₂O₂ and malondialdehyde (MDA) in leaves of hydroponically-grown perennial ryegrass (Lolium perenne) seedlings under 150 mmol/L NaCl stress were investigated. The results showed that 100 μmol/L SNP slightly reduced the MDA and H₂O₂ contents of ryegrass seedling leaves under normal growth conditions, but exogenous methylene blue (MB), the inhibitor of the key enzyme guanylate cyclase (GC) during signal transfer of NO, increased the MDA and H₂O₂ contents. Exogenous SNP significantly alleviated MDA and H₂O₂ accumulations in ryegrass seedling leaves under NaCl stress, while the activities of APX, GR and DHAR, the contents of ASA and GSH, and the ratios of ASA/DHA and GSH/GSSG were increased. Under NaCl stress, the DHA and GSSG contents were reduced while the activity of MDAR was not affected. MB reversed some effects of SNP under NaCl stress, such as increasing the levels of MDA and H₂O₂, but reducing the activities of APX and GR, the contents of ASA and GSH, and the ratios of ASA/DHA and GSH/GSSG. The DHA and GSSG contents were increased but the activities of MDAR and DHAR were not significantly affected under NaCl stress. These findings suggested that NO might be involved in regulation of ascorbate-glutathione cycle metabolism dependent on the activities of APX and GR, the contents of ASA and GSH, and the ratios of ASA/DHA and GSH/GSSG through the GC pathway thus alleviating the oxidative damage to ryegrass seedling leaves from salt stress and improving salt resistance of the perennial ryegrass plants.