63 patients of cardiac and cerebral vascular diseases were observed at 3 hours and/or one week after taking Andrographis paniculata (AP) extracts. Results showed that both 1 min. and 5 min. platelet aggregation induced by ADP were significantly inhibited (P < 0.001), 33 cases of them were observed for one week. The aggregation rate was even significantly lower than that of 3 hours. Serotonin (5-HT) release reaction from platelet was observed in 20 volunteers taking AP. Result showed 5-HT released from platelet decreased (P < 0.01), but plasma 5-HT level remained unchanged. The ultrastructural observation showed that AP could inhibit the releasing of dense and alpha agranules from platelet and dilating of canalicular system. It suggested that a raise of platelet cAMP level might be the mechanism of antiplatelet effect of AP.