In the present study, we investigated the effects of increasing soil NaCl on leaf Na⁺ and Cl^－ concentrations, O₂^-• production rate, activity of antioxidant enzymes, e.g. superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT) and glutathione reductase (GR) and the relevance to salt tolerance in 1-year-old plants of Populus euphratica (salt-resistant) and P.popularis 35-44 (salt-sensitive). Na⁺ and Cl^－concentrations in P.popularis leaves markedly increased upon increasing salinity and reached the highest level on day 18, which was 17.8- and 14.6-fold of that in controls, correspondingly, O₂^-• production rate in P.popularis leaves increased to the maximum. However, there was no significant increase of SOD activity in stressed P.popularis plants and CAT activity remained lower than control levels during the period of salt stress. APX and GR activity began to increase after 13-18 days of salt treatment when stressed P.popularis plants exhibited leaf injury symptoms. This was the typical response of salt-induced oxidative stress in P.popularis. In contrast to P.popularis, Na⁺ and Cl^－ concentration in P. euphratica leaves did not increase after salt stress was initiated, but O₂^-• production rate began to increase after 7 days of salinity. Activity of SOD, APX and CAT was elevated corresponding to the increased O₂^-• production, suggesting that P. euphratica plants were able to sense salt stress and up-regulated the antioxidant enzymes to reduce salt-induced lipid peroxidation and membrane permeability, which contributing to membrane integrity maintenance and salt tolerance in P. euphratica.