The association of maleic acid with isolated rat kidney brushborder membranes and the effect of maleic acid on uptake of amino acids and α-methyl-D-glucoside by brushborder membrane vesicles has been studied. Although maleic acid associates with the membrane, no alteration of sodium dependent transport of these substrates into the vesicles was observed. These findings indicate that the maleic acid induced Fanconi syndrome in the rat with aminoaciduria and glycosuria is not due to a direct, adverse effect on the renal tubule brushborder membrane per se .