Abstract: The effects of 0.2 μmol·L^-1 exogenous 2,4-epibrassinolide (EBR) on the growth and reactive oxygen species metabolism of cucumber seedlings (‘Jinyan 4’ cucumber) under salt-alkaline stress (30 mmol·L^-1 NaHCO₃) were examined by hydroponics method. The results showed that NaHCO₃ stress significantly induced production of O₂^-· and accumulation of H₂O₂ in leaves and roots, resulting in the increases of MDA content and electrolyte leakage. Under NaHCO₃ stress, activities of superoxide dismutase, peroxidase, catalase, ascorbate peroxidase, dehydroascorbatereductase, monodehydrodroascorbate reductase, and glutathione reductase as well as contents of ascorbic acid and glutathione were firstly increased and then decreased with progress of stress time. Exogenous EBR application significantly increased the activities of antioxidant enzymes and contents of antioxidants as well as the ratio of AsA/DHA (dehydroascorbic acid) and GSH/GSSG (L-glutathione oxidized) in leaves and roots of cucumber under NaHCO₃ stress. Such changes improved the redox hemostasis in plants, reduced the level of reactive oxygen species, and alleviated the membrane lipid peroxidation. Together, they increased the alkaline tolerance of cucumber seedlings.