High concentrations of Na+ in saline soils inhibit plant growth and reduce agricultural productivity. We report here that CaMV 35S promoter driven overexpression of the Arabidopsis thaliana SOS1 gene, which encodes a plasma membrane Na+/H+ antiporter, improves plant salt tolerance in A. thaliana. Transgenic plants showed substantial upregulation of SOS1 transcript levels upon NaCl treatment, suggesting post-transcriptional control of SOS1 transcript accumulation. In response to NaCl treatment, transgenic plants overexpressing SOS1 accumulated less Na+ in the xylem transpirational stream and in the shoot. Undifferentiated callus cultures regenerated from the transgenic plants were also more tolerant of salt stress, which was correlated with reduced Na+ content in the transgenic cells. These results show that improved salt tolerance could be achieved by limiting Na+ accumulation in plant cells.