Abstract: 4′-Methyl-7- (2-hydroxy-3-isopropylaminopropoxy)-flavone hydrochloride (SIPI-549) has inhibitory actions in a dose-dependent way on experimental thrombus formation in rats and rabbits. The thrombus wet weight (TWW) of rats was reduced after iv SIPI～549 7.5 mg/kg. In vitro, when SIPI-549 was given in final concentrations of 0.1～1.6 mg/ml in native blood, rabbit TWW was reduced by 28.9～80.5%; thrombus length (SIPI-549 0.2～1.6 mg/ml) was shortened; and thrombus formation time (SIPI-549 0.2～1.6 mg/ml) and snow storm appearance time (SIPI-549 0.4～1.6 mg/ml) were prolonged. The results of ex vivo experiment (iv SIPI～549 10 mg/kg) in rabbits were basically concordent with those in vitro.The above results show that SIPI-549 is of value as anti-thrombotic agent in vivo, in vitro or ex vivo.