In renal basal-lateral membranous vesicles, the probenecid-sensitive p-aminohippurate uptake was stimulated by alloxan. This stimulation of uptake was observed only after a lag period of 15 seconds, and it reached a maximal value after one minute. Stimulation was increased by 1 mM to 5 mM alloxan in a linear fashion. The effect was maximal and constant between 5 mM and 20 mM alloxan. Alloxan affected neither the glucose space of the vesicle nor the rate of transport or diffusion of glutamate, another organic anion. The mechanism of stimulation by alloxan was not clear. Its effect was blocked by the sulfhydryl reagent N-ethylmaleimide and weakly mimicked by H2O2, an oxidizing reagent. However, ninhydrin, a structural analogue of alloxan which reacts with sulfhydryl groups, and glucose, a neutral structural analogue of alloxan, failed to stimulate probenecid-sensitive uptake.