Abstract: AIM To observe the inhibitory effect of myricetin (Myr) and kaempferol (Kae) (components of Carthamus tinctorius L.) against washed rabbit platelet (WRP) aggregation, 5-HT release and intercellular free calcium concentration elevation induced by platelet activating factor (PAF). METHODS The platelet aggregation induced by PAF was determined with turbidity assay. The platelet 5-HT release and platelet inner free calcium concentration elevation induced by PAF were measured by OPT fluorescence assay and Fura-2/AM probe fluorescent technique respectively. RESULTS The WRP aggregation and 5-HT release induced by PAF (9.55×10^-9mol·L^-1) were both inhibited by Myr and Kae in a concentration dependent manner. WRP aggregation rate can be significantly decreased by Myr and Kae. Low concentration Kae (21.8 μmol·L^-1) showed no inhibition effect against platelet aggregation. The IC₅₀ of Myr and Kae against WRP aggregation were 17.5 μmol·L^-1 and 73.7 μmol·L^-1 respectively. It was shown by OPT fluorescence assay that Myr (7.86 μmol·L^-1) and Kae (21.8 μmol·L^-1) were not effective against platelet 5 HT release (P>0.05). The IC₅₀ of Myr was 64.1 μmol·L^-1 and that of Kae was 128 μmol·L^-1. It was also found that the platelet inner free calcium concentration elevation evoked by PAF (4.78×10^-10mol·L^-1) was inhibited by Myr and Kae. CONCLUSION The rabbit platelet activation induced by platelet activating factor can be inhibited by Myr and Kae.