Overexpression of the Arabidopsis gene AtNHX1 that codes for a vacuolar Na+/H+ antiporter in transgenic cotton significantly improved tolerance to 200 mM NaCl as measured by increases in plant height, leaf size, and leaf number. Growth of wild-type plants in 200 mM NaCl had no affect on any photosynthetic parameter measured. Interestingly, most photosynthetic parameters for the AtNHX1-expressing plants were enhanced with growth in 200 mM NaCl. Net photosynthesis vs. internal CO2 concentration curves (i.e. A-Ci curves) revealed that transgenic plants exhibited a significant increase in maximum carboxylation, maximum electron transport capacity, and rate of ribulose-1,5-bisphosphate regeneration at high CO2 levels. Chlorophyll fluorescence analysis during photosynthetic induction revealed that photochemical quenching and electron transport rates for transgenic plants were higher than those for transgenic plants grown with low NaCl (control) or those for wild-type plants grown in 200 mM NaCl; however, non-photochemical fluorescence quenching was not altered. Although proline and soluble sugar contents increased in both genotypes with growth in salt, the transgenic plants accumulated greater amounts of proline. We conclude that photosynthetic factors were not responsible for the negative effect of 200 mM NaCl on wild-type plant growth. Enhancement of photosynthesis in transgenic plants may have contributed to their better growth under salt conditions in addition to their improvement relative to wild-type in mechanisms, such as proline accumulation, to protect cells against NaCl stress.