The sodium influx into the cytosol of mesophyll protoplasts from Arabidopsis thaliana cv. Columbia, wild type, was compared with the influx into sos1-1 and nhx1 genotypes, which lack the Na⁺/H⁺ antiporter in the plasma membrane and tonoplast, respectively. Changes in cytosolic sodium and calcium concentrations upon a 100 mM NaCl addition were detected by use of epifluorescence microscopy and the sodium-specific fluorescent dye SBFI, AM, and calcium sensitive Fura 2, AM, respectively. There was a smaller and mainly transient influx of Na⁺ in the cytosol of the wild type compared with the sos1-1 and nhx1 genotypes, in which the influx lasted for a longer time. Sodium chloride addition to the protoplasts’ medium induced a significant increase in cytosolic calcium concentration in the wild type at 1.0 mM external calcium, and to a lesser extent in nhx1, however, it was negligible in the sos1-1 genotype. LiCl inhibited the cytosolic calcium elevation in the wild type. The results suggest that the salt-induced calcium elevation in the cytosol of mesophyll cells depends on an influx from both internal and external stores and occurs in the presence of an intact Na⁺/H⁺ antiporter at the plasma membrane. The Arabidopsis SOS1 more effectively regulates sodium homeostasis than NHX1.