The effect of salt exposure on the expression and catalytic properties of the vacuolar H+-ATPase (V-ATPase) of carrot cells was investigated. Dark-grown suspension-cultured cells were exposed to 100 mM NaCl. This moderate salt stress retarded the transition from the lag-phase to the early exponential growth phase. It caused a sustained increase of total RNA and soluble protein, and a transient increase of membrane protein in the tonoplast-enriched fraction. Comparison of H+-transport rates with ATP hydrolysis rates revealed differential effects of salt treatment on both activities. Four days after the onset of salt exposure, the specific activity of H+-transport was increased by 60%, whereas ATP hydrolysis rate was decreased by 20%. The changes resulted in an apparent doubling of the coupling ratio of H+-transport to ATP hydrolysis, which was maintained throughout the growth period (14 d). In the tonoplast-enriched membrane fraction of salt-treated cells H+-transport activity of the vacuolar H+-pyrophosphatase and the rate of passive dissipation of an imposed pH-gradient remained unchanged as compared to control cells. Western blot analysis did not reveal significant salt-induced changes in the amounts of subunits A, B, D, E, and c, whereas transcript levels for subunits A and c were modulated during the growth period, the former showing a modest and transient increase in response to salt exposure. The results indicate that in salt-treated carrot cells the intrinsic properties of the V-ATPase are affected by an as yet unknown mechanism leading to an increased pump efficiency, whereas the amount of the holoenzyme is not affected.