Astragaloside IV is the major active constituent of Astragalus membranaceus, which has been widely used for the treatment of cardiovascular diseases in China. However, the effects of astragaloside IV on myocardial ischemia and its mechanisms of action remain largely unknown. In this study, we have examined the effects of astragaloside IV on myocardial infarction and coronary flow in vivo and in vitro. The possible roles of its antioxidative and nitric oxide-inducing properties were also explored. Astragaloside IV significantly reduced infarct size in dogs subjected to coronary ligation in vivo. Astragaloside IV also improved post-ischemic heart function and ameliorated reperfusion arrhythmias in rat hearts in vitro. The cardioprotection of astragaloside IV was accompanied by a significant increase in coronary flow both in vivo and in vitro. The nitric oxide synthase inhibitor, Nomega-nitro- L-arginine methyl ester partially abrogated astragaloside IV's protective effect on heart function. Myocardial antioxidative enzyme superoxide dismutase activity increased with astragaloside IV administration. These data suggest the potential roles of antioxidative and nitric oxide-inducing properties of astragaloside IV in its protection from myocardial ischemia.