Wistar rats of both sex were used. Ros A was intravenously injected 5-10 min before blood collection or the ligation of vena cava. 1. Stasis-induced venous thrombosis: A tight ligature was applied to inferior vena cava below the left renal vein in anesthetized rats. The abdominal walls were closed and then reopened two hours later. The vena cava was clamped 2 cm below the ligature. This segment was cut to remove the thrombus. The dry weight of the thrombus was determined. 2. Platelet aggregation: Using Born's method the platelet aggregation induced by collagen or ADP was studied. 3. Blood coagulation times: Blood recalcium time (RT), kaolin partial thromboplastin time (KPTT) and prothrombin time (PT) were estimated. 4. Plasma fibrinolytic activity was observed by the determination of euglobulinolytic time (ELT). Plasma fibrinogen content was estimated based on the biuret reaction. The venous thrombosis was inhibited by 41.9% and 54.8% (P < 0.05) when Ros A was injected at the dosages of 50 and 100 mg/kg. The blood platelet aggregation elicited by collagen was suppressed by 30.4% (P < 0.05) and 46.4% (P < 0.01) after the injection of Ros A at doses of 100 and 150 mg/kg respectively. The ELT was shortened after the injection of Ros A (100 and 150 mg/kg) as compared with the control value (P < 0.05), while the plasma fibrinogen content remained unchanged. The results that Ros A showed mild antithrombotic effect. The mechanism of this effect might be related to its inhibition of platelet aggregation and promotion of fibrinolytic activity.