Sodium (Na+) uptake into the halophyte quinoa (Chenopodium quinoa Willd.) plants was compared with the uptake into pea (Pisum sativum L.), and related to changes in cytosolic pH and potassium (K+) concentration in plant tissues. The total uptake of Na+ and K+ in roots and shoots was analyzed and compared with net ion fluxes at the root xylem parenchyma, determined by ion-specific microelectrodes in a non-invasive way. The cytosolic changes of Na+ concentration, [Na-cyt(+)], and pH, pH(cyt), were measured by fluorescent probes, specific to Na+ and H+, using a dual-wavelength fluorescence microscopy. These changes were monitored in protoplasts after cultivation with or without 100 mM NaCl, and after addition of NaC1 to the protoplasts. Roots and shoots of quinoa controls contained much higher K+ levels than pea roots and shoots, and the K+ levels increased even more after salinity treatments in quinoa. The cytosolic uptake of Na+ in quinoa protoplasts was transient if less than 200 mM NaCl was added, while in pea the Na+ concentration increased even upon addition of 50 mM Na+ and gradually increased with time. Saline conditions during cultivation increased pH(cyt) of both species. However, with a direct addition of NaCl to control protoplasts only a small increase was seen in pea pH(cyt) while in quinoa this increase was much larger. The different reactions of pH(cyt) to salinity when NaCl was added to salinity-treated seedlings may reflect an increased proton pump activity in quinoa, while this activation is lacking in pea. ABA addition to the root xylem parenchyma cells induced a net efflux of K+ and acidification of the xylem. On the other hand, 20 mM NaC1 addition induced a net flux of protons in both species, and a net K+ flux in pea, but not in quinoa, probably since such a low concentration is not a stress for quinoa. It is suggested that salinity tolerance in quinoa is achieved by a faster removal of Na+ from the cytosol and a high K+ concentration in roots and shoots under salinity, resulting in a high K+/Na+ ratio, and that this mechanism is driven by a higher proton pump activity, compared with glycophytic pea species.