Auxin effects on the H+-ATPase promoted proton translocation in purified tobacco leaves plasma membrane vesicles were investigated according to the hormone concentration. Addition of IAA at low auxin concentrations was shown to induce a stimulation of the H+ translocation. However, an opposite effect was also observed at higher concentrations. In addition, auxin concentrations inducing both effects appeared to depend dramatically on the plant age: at the end of the induction to flowering period, the sensitivity is nearly 1000-fold higher than that of vegetative or flowering plants. Thus proton translocation may be either stimulated or inhibited by auxins, according to the hormone concentration and/or the physiological state of the plant.