We studied the effects of propranolol on infarct size and hemodynamic impairment induced by 24 h-coronary ligation. The myocardial infarction produced by the left circumflex coronary artery ligation was more consistent than that induced by the left anterior descending coronary artery ligation, suggesting that the former is a more appropriate experimental model for pharmacological evaluations. Oral treatment with propranolol, 3-30 mg/kg, reduced infarct size and reduced the elevated left ventricular end-diastolic pressure, which was shown to be most closely related with infarct size, in dogs with circumflex coronary artery ligation extending over 24 h. In conclusion, our results indicate that propranolol protects against the enlargement of infarct size and improves the impaired hemodynamics observed in myocardial infarcted dogs with occlusions extending over 24 h, as well as in dogs with less than 24 h-occlusions reported by numerous investigators.