Tanshinone IIA (tan), a derivative of phenanthrenequinone, is one of the key components of Salvia miltiorrhiza Bunge. Previous reports showed that tan inhibited the apoptosis of cultured PC12 cells induced by serum withdrawal or ethanol. However, whether tan has a cardioprotective effect against apoptosis remains unknown. In this study, we investigated the effects of tan on cardiac myocyte apoptosis induced both by in vitro incubation of neonatal rat ventricular myocytes with H(2)O(2) and by in vivo occlusion followed by reperfusion of the left anterior descending coronary artery in adult rats. In vitro, as revealed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium (MTT) assay, treatment with tan prior to H(2)O(2) exposure significantly increased cell viability. Tan also markedly inhibited H(2)O(2)-induced cardiomyocyte apoptosis, as detected by ladder-pattern fragmentation of genomic DNA, chromatin condensation, and hypodioloid DNA content. In vivo, tan significantly inhibited ischemia/reperfusion-induced cardiomyocyte apoptosis by attenuating morphological changes and reducing the percentage of terminal transferase dUTP nick end-labeling (TUNEL)-positive myocytes and caspase-3 cleavage. These effects of tan were associated with an increased ratio of Bcl-2 to Bax protein in cardiomyocytes, an elevation of serum superoxide dismutase (SOD) activity and a decrease in serum malondialdehyde (MDA) level. Taken together, these data for the first time provide convincing evidence that tan protects cardiac myocytes against oxidative stress-induced apoptosis. The in vivo protection is mediated by increased scavenging of oxygen free radicals, prevention of lipid peroxidation and upregulation of the Bcl-2/Bax ratio.