In 16 dogs the endothelium of the left anterior descending coronary artery was injured mechanically. Then a copper wire was inserted into the lumen as a choke of flow and the vessel was slightly compressed from outside by a constrictor. Eight dogs had been treated beforehand with a preparation of flavone extracted from the root of the Chinese medicinal herb Andrographis paniculata (TFAP). In the control group, saline solution was given, the epicardially recorded ST segment started to elevate within 15 minutes, the platelet aggregation rate and the plasma levels of TXB2 increased rapidly, whereas the level of 6-k-PGF1 alpha remained stable. Platelet cGMP rose continuously; however, platelet cAMP rose only at 60 minutes. Histological findings confirmed the occurrence of arterial thrombus and myocardial necrosis. Contrariwise, in the pretreated group there was no elevation of the ST segment, plasma 6-k-PGF1 alpha and platelet cAMP were increased, the production of TXB2 and aggregation of platelets were inhibited, and no thrombus or myocardial infarction was induced. All data suggest that TFAP might promote the synthesis of PGI2, inhibit the production of TXA2, stimulate the synthesis of cAMP in platelets, impede aggregation of platelets, and thereby prevent the formation of thrombi as well as the development of myocardial infarction.