One hour after the development of myocardial infarction by formation of thrombus, watery extract of Andrographis paniculata Nees (APN) was injected intravenously to 7 dogs for observing the protective effect of this drug on ischemic myocardium, 6 dogs served as control group. In the treated group PGI2 increased remarkably, synthesis of TXA2 was inhibited, cAMP in platelets was elevated, CK-MB peak was lowered and appeared earlier, ELT was shortened, release of platelet beta-GT was decreased, platelet maximum aggregation rate was inhibited, the size of ischemic area recorded by epicardial ECG was reduced, the amplitude of ST segment elevation was lower than that in the control group, Q wave appeared in only one dog. Pathologically, the myocardial structure surrounding the initially appearing ischemic area, i.e, the reversibly damaged area, became relatively normal, while the degree of myocardial degeneration and necrosis in the central part of the ischemic area was mild. These data suggest that APN may limit the expansion of ischemic focus, exert marked protective effect on reversibly ischemic myocardium and demonstrate a weak fibrinolytic action.