To evaluate the extent to which the protective effect of metoprolol was accompanied by changes in myocardial oxygen consumption and metabolism, thrombotic occlusion of coronary artery followed by infusion of metoprolol or placebo was performed in twenty four German Shepherds. To restore a coronary blood flow rt-PA was administered. Plasma levels of oxygen, glucose, lactic acid, non esterified fatty acids, triacylglyceride and adenosine breakdown products were measured before and at the end of the occlusion and in the early and late reperfusion periods. Regional myocardial blood flow was measured by means of radioactive tracer microspheres. Infarct size was estimated after perfusion and staining of excised hearts with Evans blue. Plasma levels of metoprolol were determinated before the end of occlusion and during reperfusion and therapeutic concentrations were confirmed. The infarct size was smaller in dogs receiving metoprolol (21.6 +/- 20.7 vs 43.0 +/- 17.3% p. < 0.02). Coronary collateral blood flow was greater in metoprolol than in placebo dogs (18.68 +/- 7.58 vs 11.05 +/- 6.10 ml/min/100g, p. < 0.01). As a consequence of myocardial ischemia a shift toward carbohydrate utilization, the myocardial lactate release and the accompanying symptoms of diminished myocardial lipid uptake were observed. A washout of adenosine degradation products during early reperfusion was also noticed. In beta 1 blocked animals the reduction of myocardial oxygen consumption and preserved myocardial uptake of lactate and non esterified fatty acids were documented.