The initial rate of (2-(14)C)acetic acid (AA) uptake by corn roots was used for probing the dependency of the root cell surface pH on H(+) excretion. AA influx was linearly related to AA concentration, dependent on the concentration of the undissociated form (AH), unaffected by variations of the membrane potential, and was thus assumed to result mainly from the free diffusion of AH across the membrane. Various treatments (vanadate, dicyclohexylcarbodiimide, hypoxia, nitrate, root ageing, fusicoccin) were used to vary H(+) flux while the medium pH was maintained constant. There was a positive relation between AA influx and the net H(+) efflux. This relation disappeared when the proton buffering strength of the absorption medium was increased. These results indicate that the pH at the membrane surface was lowered by H(+) excretion, even in situations where the bulk (pH) was unaffected. The depressive effect of vanadate on AA influx was counteracted by acidifying the medium in order to estimate this pH shift: -1.2 pH units in 12.5 millimolar K(2)SO(4) (pH 6.8). Substituting AA by butyric acid showed that this estimation was not dependent of the probe used.