Background/Aims: Renal secretion of organic anions is critically dependent on their basolateral uptake against the electrochemical gradient. Due to their localization, two transporters are likely involved, namely OAT1 and OAT3. While OAT1 as an exchanger clearly operates in the secretory direction, OAT3 in its previously supposed mode as a uniporter should move anionic substrates from cell to blood. It would thus dissipate gradients established by OAT1 of common OAT1/OAT3 substrates. In the present study we therefore reinvestigated the driving forces of human OAT3. Methods: The human OAT3 obtained from the Resource Center/Primary Database was made functional by site-directed mutagenesis. Using the Xenopus laevis oocyte expression system, hOAT3-mediated transport of estrone sulfate (ES) and dicarboxylates was assayed for cis-inhibition and/or trans-stimulation in both the uptake and efflux direction. Results: hOAT3-mediated efflux of glutarate (GA), can be significantly trans-stimulated by a variety of ions with high cis-inhibitory potency, including GA (282%), α-ketoglutarate (476%), p-aminohippurate (179%), and, most notably, urate (167%). Urate cis-inhibited ES uptake with an IC₅₀ close to normal serum urate concentrations. Conclusion: These data indicate that OAT3 does not represent a uniporter but operates as an organic ion%dicarboxylate exchanger similar to OAT1, and may mediate renal urate secretion.