Abstract: It was shown with turbidimetry that tetrapyrazine can inhibit platelet aggregation. Tetrapyrazine was found to increase the membrane fluidity of platelets using fluorescence polarization. On the contrary, no effect was found on the membrane fluidity of lioosomes of various constituents and that of the erythrocytes membran. These results suggest that inhibition was not caused by affecting the membrane lipid directly. Our experiment also showed that tetrapyrazine could raise the electrophoretic mobility of platelet, that is, increase its surface charge density. Besides, tetrapyrazine may increase the fluorescence intensity of ANS. a fluorescence probe labeled on the surface of platelet membrane. The possibility that tetrapyrazine may have the ability to substitute Ca²⁺ ions of platelet membrane is discussed.