The effects of sophoridine on acute myocardial infarction induced by ligation of left anterior descending coronary artery in 16 anesthetized and open-chest dogs were studied. Treatment with sophoridine 20 mg.kg-1 iv reduced the size of myocardial infarct 6 h after ligation and lowered the elevated plasma levels of lactic acid and pyruvic acid 15, 30 and 60 min after ligation. Sophoridine also lowered the activity of serum CPK 60 and 120 min after ligation and reduced TXB2 and increased 6-keto-PGF1 alpha, hence the metabolites of PGI2/TXA2 ratio after ligation was increased. Electron microscopy showed that sophoridine protected mitochondria of ischemic cells. These results indicated that sophoridine showed protective effects on acute myocardial infarction. The beneficial effect on PGI2/TXA2 and mitochondria seems to contribute its protective action.