Quercetin (3,3',4',5,7-pentahydroxyflavone) is an attractive therapeutic flavonoid for cancer treatment because of its beneficial properties including apoptotic, antioxidant, and antiproliferative effects on cancer cells. However, the exact mechanism of action of quercetin on ion channel modulation is poorly understood in bladder cancer 253J cells. In this study, we demonstrated that large conductance Ca2＋- activated K＋ (BKCa) or MaxiK channels were functionally expressed in 253J cells, and quercetin increased BKCa current in a concentration dependent and reversible manner using a whole cell patch configuration. The half maximal activation concentration (IC50) of quercetin was 45.5±7.2ՌM. The quercetin-evoked BKCa current was inhibited by tetraethylammonium (TEA; 5 mM) a non-specific BKCa blocker and iberiotoxin (IBX; 100 nM) a BKCa-specific blocker. Quercetin-induced membrane hyper</SUB>polarization was measured by fluorescence-activated cell sorting (FACS) with voltage sensitive dye, bis (1,3-dibutylbarbituric acid) trimethine oxonol (DiBAC4(3); 100 nM). Quercetin-evoked hyperpolarization was prevented by TEA. Quercetin produced an antiproliferative effect (30.3±13.5%) which was recovered to 53.3±10.5% and 72.9±3.7% by TEA and IBX, respectively. Taken together our results indicate that activation of BKCa channels may be considered an important target related to the action of quercetin on human bladder cancer cells.