A number of flavonoids have been reported to have beneficial effects on cardiovascular system. Flavonoids modulate vascular tone through endothelium-dependent or endothelium-independent relaxation. Although a few mechanisms for endothelium-independent relaxation were suggested, such as interference with protein kinase C or phosphodiesterase and inhibition of the influx of extracellular Ca2+, however, the mode of action of flavonoids remains controversial. We hypothesized that relaxation of vascular tone by flavone involves inhibition of Rho kinase (ROCK) pathway. Rat aortic rings were denuded of endothelium, mounted in organ baths, and subjected to U46619. Flavone dose-dependently inhibited the U46619-induced contractile response and myosin light chain (MLC20) phosphorylation. U46619 (10-7 M) induced vascular contraction by phosphorylation of MYPT1 at Thr850 but not at Thr696. Incubation with flavone for 30 min decreased A phosphorylation of MYPT1Thr850, but MYPT1Thr696. These results suggest that flavone exerts vasodilator effects, at least in part, through inhibition of Rho kinase pathway.