Studies on the effects of flavonoids isolated from the roots of Scutellaria baicalensis on the fibrinolytic system induced by trypsin in cultured human umbilical vein endothelial cells (HUVECs) showed that baicalein (1) strongly inhibited the reduction of t-PA production and the elevation of PAI-1 production induced by trypsin. The IC50 for PAI-1 production was 3.7 microM. In addition, wogonin (3), oroxylin A (5), skullcapflavone II (6), and 2',5,5',7-tetrahydroxy-6',8-dimethoxyflavone (7) inhibited the elevation of PAI-1 induced by trypsin, though less strongly; their IC50 were 105, 61, 110, and 88 microM, respectively. These findings suggest that baicalein prevents the thrombotic tendency induced by trypsin.