TaNHX2, a vacuolar Na+-H+ antiport gene from wheat (Triticum aestivum L.), was transformed into alfalfa (Medicago sativa L.) via Agrobacterium-mediated transformation to evaluate the role of vacuolar energy providers in plant salt stress responses. PCR and Southern blotting analysis showed that the target gene was integrated into the Medicago genome. Reverse transcription-PCR indicated that gene TaNHX2 was expressed at the transcriptional level. The relative electrical conductivity in the T-2 transgenic plants was lower and the osmotic potential was higher compared to the wild-type plants under salt stress conditions. The tonoplast H+-ATPase, H+-pyrophosphatase (PPase) hydrolysis activities and ATP-dependent proton pump activities in transgenic plants were all higher than those of wild-type plants, and the enzyme activities could be induced by salt stress. The PPi-dependent proton pump activities decreased when NaCl concentrations increased from 100mM to 200mM, especially in transgenic plants. The vacuolar Na+-H+ antiport activities of transgenic plants were 2-3 times higher than those of the wild -type plants under 0mM and 100mM NaCl stress. Na+-H+ antiport activity was not detectable for wild-type plants under 200mM NaCl, but for transgenic plants, it was further increased with an increment in salt stress intensity. These results demonstrated that expression of the foreign TaNHX2 gene enhanced salt tolerance in transgenic alfalfa.