The effect of anions on Na+-cotransport of succinate, lactate, glucose, and phenylalanine was studied under voltage clamped conditions in brush-border membrane vesicles prepared from rabbit renal cortex. The initial rate of succinate uptake varied by an order of magnitude depending on the anion: the highest rates were obtained with fluoride and gluconate, and the lowest with iodide. The anion sequence corresponded with the inverse of the anion hydration energies. The kinetics of succinate uptake were measured in the presence of fluoride and chloride. There was no difference in the maximal rates of uptake, but the Kt in fluoride (0.30 mM) was less than half that in chloride (0.70 mM), i.e. Cl- behaved as a competitive inhibitor of succinate transport with a Ki of 150 mM. The uptake of L-lactate, D-glucose and L-phenylalanine was less sensitive to anions, and there was no correlation with hydration energies. We conclude that the anion effects on sugar and amino acid uptakes measured under open-circuit conditions are largely due to variations in membrane potential, but in the case of the dicarboxylate transporter anions behave as weak competitive inhibitors. The specificity of the anion inhibition suggests that the dicarboxylate binding sites have a weak field strength relative to water.