Abstract: Protocatechualdehyde is one of the constituents of Ilex chinensis Sims and Salvia mitirrhiza Bunge. In previous studies we have shown that protocatechualdehyde possesses effects of inhibiting both platelet aggregation and serotonin release. In the present paper, the effect of in vitro addition of protocatechualdehyde to rabbit platelets was studied by measuring fluorescence polarization of DPH (1,6-diphenyl-1,3,5,-hexatriene) labelled platelet membranes. The results showed that the fluorescence polarization was increased in comparison with nonprotocateehualdehyde treated control platelets at doses which cause inhibition of platelet aggregation. It would appear that protocatechualdehyde may induce a decrease of the fluidity of platelet membrane.