The aim of the present report was to study the effect of ischemia-reperfusion on the endothelial cell function of coronary vessels. Twelve adult male dogs were instrumented for the measurement of aortic and left ventricular pressures, heart rate and coronary blood flow. The left anterior descending coronary artery was occluded for 90 minutes followed by 20 minutes of reperfusion. Acetylcholine was infused into the coronary artery at a rate of 15 mu g/kg/min. Coronary flow, heart rate and aortic and left ventricular pressures were registered during the pre-occlusion period and after 20 minutes of reperfusion under basal conditions, as well as during acetylcholine administration. These same parameters were also measured during reactive hyperemia following vessel deocclusion. Acetylcholine produced a 155 % increasing coronary flow during the pre-occlusion period (p < 0.05). In the reperfusion period, no statistically significant difference was observed between the flows in the presence and absence of this substance, nor were there any differences in the other cardiovascular parameters monitored. Triphenyltetrazolium staining confirmed myocardial infarction in all the hearts examined. The authors conclude that reperfusion following myocardial infarction prevents the increasing in coronary flow in response to acetylcholine as a result of endothelial dysfunction in the resistance coronary vessels.4941671147151