In allopurinol-allergic patients, uricosuric agents are often used in the treatment of hyperuricemia. The existing uricosuric agents are not without problems and the availability of better and safer alternatives is highly desirable. Our previous study (J Pharmacol Exp Ther (2006) 316:169-175) has demonstrated that morin (3,5,7,2',4'-pentahydroxyflavone), which occurs in the twigs of Morus alba L. documented in traditional Chinese medicinal literature for treatment of conditions akin to gout, is a potent inhibitor of urate uptake in rat renal brush-border membrane vesicles. It is also effective in lowering uric acid level in a hyperuricemic rat model in vivo. Whether morin is an equally effective uricosuric agent in human requires verification. The human urate anion transporter (hURAT1) has recently been cloned and identified to be the organic anion transporter that mediates renal urate reabsorption in the human kidney. In the present investigation, human embryonic kidney cells were transfected with hURAT1 and the expression was validated by reverse transcription-polymerase chain reaction and subcellular distribution of the exogenously introduced transporter by confocal microscopy. The inhibitory actions of morin on human renal urate reabsorption were demonstrated using this system. The IC50 value of the inhibition by morin was determined to be 2.0 microM, compared with 50 microM for probenecid, 100 microM for sulfinpyrazone, and 0.3 microM for benzbromarone. Kinetic analysis of the uptake inhibition by morin indicates that this compound is a competitive inhibitor of urate uptake on the human urate transporter with a K(i) value of 5.74 microM.