The effects of dl-propranolol, d-propranolol, dl-pindolol and dl-practolol on regional myocardial blood flow (assessed by means of tracer microspheres) and on ST-segment elevation in ischemic and nonischemic areas of the canine left ventricle have been investigated. dl-Propranolol and dl-pindolol, but not dl-practolol and d-propranolol, induced blood flow redistribution from the epicardium to the endocardium both in ischemic and nonischemic areas. dl-Propranolol-induced redistribution was abolished by atrial pacing at the control heart rate value. These results indicate that the redistribution phenomenon only occurs if both a bradycardia-inducing beta1 adrenoreceptor blockade and a coronary vessels beta2 adrenoceptor blockade are simultaneously achieved. All four drugs significantly decreased ST-segment elevation in ischemic areas. Under atrial pacing, this effect was abolished with dl-practolol but only reduced with dl- and d-propranol, suggesting that, besides bradycardia, membrane stabilization might be involved in protection against ST-segment elevation in ischemic areas.