Using the scanning ion-selective electrode technique, fluxes of H+, Na+, and Cl- were investigated in roots and derived protoplasts of salt-tolerant Populus euphratica and salt-sensitive Populus popularis 35-44 (P. popularis). Compared to P. popularis, P. euphratica roots exhibited a higher capacity to extrude Na+ after a short-term exposure to 50 mM NaCl (24 h) and a long term in a saline environment of 100 mM NaCl (15 d). Root protoplasts, isolated from the long-term-stressed P. euphratica roots, had an enhanced Na+ efflux and a correspondingly increased H+ influx, especially at an acidic pH of 5.5. However, the NaCl-induced Na+/H+ exchange in root tissues and cells was inhibited by amiloride (a Na+/H+ antiporter inhibitor) or sodium orthovanadate (a plasma membrane H+-ATPase inhibitor). These results indicate that the Na+ extrusion in stressed P. euphratica roots is the result of an active Na+/H+ antiport across the plasma membrane. In comparison, the Na+/H+ antiport system in salt-stressed P. popularis roots was insufficient to exclude Na+ at both the tissue and cellular levels. Moreover, salt-treated P. euphratica roots retained a higher capacity for Cl- exclusion than P. popularis, especially during a long term in high salinity. The pattern of NaCl-induced fluxes of H+, Na+, and Cl- differs from that caused by isomotic mannitol in P. euphratica roots, suggesting that NaCl-induced alternations of root ion fluxes are mainly the result of ion-specific effects.