Abstract: Quercetin, a flavonoid widely distributed in plants, had been reported to inhibit platelet aggregation and secretion. The mechanisms of the quercetin effect on platelet function have not been completely understood. In the present paper, the effects of quercetin on platelet aggregation and ³H-5HT release induced by thrombin, ADP and PAF were studied; the effect of quercetin on the fluidity of platelet membrane lipids was also studied by means of technique of fluorescence polarization quercetin was found to inhibit platelet aggregation induced by ADP, thrombin (rat) and PAF (rabbit). Platelet aggregation was almost completely inhibited when drug concentration was increased to 150 μmol. for PAF and 600 μmol. for thrombin. However, when quercetin concentration was increased to 1200 μmol. platelet aggregation induced by ADP was inhibited uncompletely. The ³H-5HT content of platelet (rabbit) was decreased by quercetin to 28.3% to 53.1% in a range of concentration of 75 μmol. to 1200 μmol. The degree of fluorescence polarization was increased significantly by quercetin in a drug concentration as low as 30 μmol. (P<0.005) and increasing the drug concertrations was followed by increasing the degree of fluorescence polarization continously. This indicates that the fluidity of platelet membrane lipids could be significantly decreased by quercetin.We Suggest that quercetin may be a valualbe inhibitor of platelet functions. Decrease of the fluidity of platelet membrane lipids may be the mechanisms of querctin action, but further study will be necessity to confirm it.