The allelopathic effect of some benzoic acid (BA) OH- and OCH3-ring substituents was studied on cucumber root transmembrane potential difference (Vm). Most of the methoxy-BAs induced a rapid Vm depolarization, followed by a Vm hyperpolarization, with the only exception for p-anisic acid (pA). On the other hand, salicylic acid (SA) and 3,4-dimethoxybenzoic acid (DHB) strongly depolarized Vm. A positive correlation was found between Vm hyperpolarization and lipophilicity of methoxylated BAs, whereas a positive correlation was found between lipophilicity and Vm depolarization of hydroxylated BAs. The influence of BAs on K was studied by means of specific blocking with Cs indicating a possible direct interaction of SA, gallic acid (GA), vanillic acid (VA) and 3,4-dimethoxybenzoic acid (DMB). Interference of BAs with the Vm hyperpolarizing effect of root perfusion with the fungal toxin fusicoccin were also observed.