Capsaicin was found to be a potent inhibitor of platelet aggregation and release reaction. It inhibited the aggregation of rat platelet induced by collagen and thrombin, but only slightly reduced those of AA and A23187. The IC50 on collagen-induced platelet aggregation was about 85 micrograms/ml. Less inhibition was observed in the aggregation of platelet-rich plasma. Increase of the calcium concentration could not overcome the inhibitory effect. Washing of the capsaicin-pretreated platelets only partially reversed the inhibition. Capsaicin also inhibited ATP release induced by thrombin and A23187 in the presence of EDTA. MDA and TXB2 formation were markedly inhibited by capsaicin in platelets challenged by collagen, thrombin and A23187. In AA-stimulated platelets, MDA formation was slightly decreased and TXB2 formation was not affected. Capsaicin showed more marked inhibition in the presence of CP/CPK, indomethacin or a combination of both. Capsaicin reduced the hemolysis of RBCs induced by hydrogen peroxide or hypotonicity. It was concluded that capsaicin had some membrane stabilizing property and this might lead to the interference of the activation of phospholipase A2.