Ilexonin A is an effective compound isolated from Ilex pubescens Hook. et Arn. It has been used in the clinic to treat cardiovascular diseases such as cerebral embolism and myocardial infarction. Experimental studies have shown that it inhibited thrombosis, platelet adhesion, and platelet aggregation and 5-HT release induced by collagen, ADP or A23187 in vitro and in vivo. The inhibitory effect of platelet aggregation induced by A23187 was reduced by adding CaCl2 (1mM) to the medium. It suggests that the effects of llexonin A on platelet function are possibly related to calcium. The present experiments were designed to investigate the effects of Ilexonin A on thrombin-induced Ca2+ fluxes of platelets, in which quin-2 was used to measure the cytoplasmic Ca2+ concentration. The results indicated that the free calcium concentrations of resting platelets and the platelets activated by thrombin were 78.65 +/- 7.74 nM and 871.10 +/- 123.63 nM respectively. After adding EGTA (lmM) to the medium, the calcium concentrations reduced to 34.75 +/- 7.77 nM and 50. 86 +/- 7.44 nM. Ilexonin A and verapamil markedly inhibited the thrombin induced Ca2+ influx. The IC50 was 76.8 microM and 67.5 microM respectively. But both had no effect on thrombin-induced Ca2+ release from dense tubular system. It suggests that Ilexonin A acts most likely as a calcium slow channel blocker.