In the first phase of salt stress, growth of plants is impaired mainly by osmotic stress. To elucidate the effect of NaCl salinity on elongation growth of maize leaves in the first phase of salt stress, we investigated the effect of NaCl on gene expression and activity of the plasmalemma H+ ATPase of elongating leaves of maize (Zea mays L.). Treatment of maize plants with 125 mM NaCl for 3 d decreased leaf growth relative to control plants (1 mM NaCl). Whereas H+ ATPase hydrolytic activity was unaffected, the ability of the H+ ATPase to establish a pH gradient was strongly reduced. Total mRNA of plasmalemma H+ ATPase was slightly increased. However, mRNA of the ATPase isoform MHA1 was significantly reduced and ATPase isoform MHA4 was strongly increased at the mRNA level. Synthesis of total H+ ATPase protein was unchanged as revealed by western blot. The results indicate that reduced pumping of H+ ATPase in leaf plasmalemma under salt stress may be caused by a switch to gene expression of the specific isoform MHA4, which shows inferior H+-pumping efficiency in comparison to isoforms expressed under control conditions. We propose that reduced H+ pumping of plasmalemma H+ ATPase is involved in the reduction of leaf growth of maize during the first phase of salt stress.