Effects of quercetin, a kind of flavonoids, on the vasodilating actions were investigated. Among the mechanisms for quercetin-induced vasodilatation in rat aorta, the involvement with the Ca2＋ activated K＋ (KCa) channel was examined. Pretreatment with NE (5ՌM) or KCl (60 mM) was carried out and then, the modulation by quercetin of the constriction was examined using rat aorta ring strips (3 mm) at 36.5oC. Quercetin (0.1 to 100ՌM) relaxed the NE-induced vasoconstrictions in a concentration- dependent manner. NO synthesis (NOS) inhibitor, NG-monomethyl-L-arginine acetate (L-NMMA), at 100ՌM reduced the quercetin (100ՌM)-induced vasodilatation from 97.8±3.7% (n=10) to 78.0±11.6 % (n=5, p＜0.05). Another NOS inhibitor, L-NG-nitro arginine methyl ester (L-NAME), at 100ՌM also had the similar effect. In the presence of both 100ՌM L-NMMA and 10ՌM indomethacin, the quercetin-induced vasodilatation was further attenuated by 100ՌM tetraethylammonium (TEA, a KCa channel inhibitor). Also TEA decreased the quercetin-induced vasodilatation in endothelium-denuded rat aorta. Used other KCa channel inhibitors, the quercetin-induced vasodilatation was attenuated by 0.3ՌM apamin (a SK channel inhibitor), but not by 30 nM charybdotoxin (a BK and IK channel inhibitor). Quercetin caused a concentration-dependent vasodilatation, due to the endothelium- dependent and -independent actions. Also quercetin contributes to the vasodilatation selectively with SK channel on smooth muscle.