[2-14C]Urate uptake and efflux were studied in brush border and basolateral membrane vesicles of rat renal cortex that were exposed to 20 microM copper chloride. In the presence of inwardly directed NaCl gradients urate uptake was maintained at levels in excess of chemical equilibrium. Comparison of glucose and chloride uptakes revealed that equilibrium glucose uptake was not affected by copper, but chloride failed to reach equilibrium in copper-exposed vesicles. It is suggested that the persistence of an electrolyte gradient could provide a driving force to raise the concentration of free intravesicular urate above that in the media. Preincubation of vesicles with unlabeled urate failed to diminish uptake of added urate; rather, urate uptake was trans stimulated. Uptake of labeled urate was also significantly accelerated when an outward gradient for unlabeled urate was created. Pyrazinoic and oxonic acids also trans stimulated urate uptake. The demonstration of accelerated homeo- and heteroexchange diffusion indicates that transport is carrier mediated in both brush border and basolateral vesicles. Outwardly directed hydroxyl gradients failed to influence urate uptake in either the presence or absence of copper or NaCl. Thus, this carrier, which is active only in the presence of trace amounts of copper, is distinct from a urate/anion exchanger.