The influence of 12 naturally occurring phenolic acids on the uptake of potassium by barley (Hordeum vulgare L. cv. Karlsberg) was examined using 86Rb-labelled potassium solutions. Without exception, all compounds tested, including both cinnamic acid and benzoic acid derivatives, caused a significant reduction of uptake. In addition, the inhibitory capacities of 15 variously substituted benzoic acids were determined over a range of different concentrations of inhibitors. For each of these compounds an inhibition constant (Ki), defined as the concentration required to produce a 50 per cent reduction of uptake, was determined. The Ki values were found to be strongly correlated with the octanol-water partition coefficients of the compounds under examination. It is clear from this study and previous work upon phosphate uptake that phenolic acids exert a generalized inhibitory effect upon active ion-uptake which is almost certainly mediated through reversible alterations in membrane permeability.