Whereas isolated heart preparations or anesthetized animals have been used to assess the actions of beta-adrenoceptor antagonists upon reperfusion-induced arrhythmias, this study evaluated the possible antiarrhythmic effects of beta-adrenoceptor antagonism in conscious animals. Conscious, chronically instrumented dogs were subjected to a temporary occlusion of the left anterior descending coronary artery for 4 h. After the first hour of reperfusion, flestolol (N-(1,1-dimethyl-2- ureidoethyl)-2-hydroxy-3-(O-fluorobenzoyloxy)-propylamine sulfate), an ultra short-acting beta-adrenoceptor antagonist, was administered i.v. at infusion rates of 1 and 2.6 micrograms/kg per min. A control group received the equivalent volume of saline during this period. Delayed reperfusion-induced arrhythmias were evaluated by using a computerized analysis system. Treatment with flestolol did not affect the total number of beats, the number of normal and ectopic beats, and the arrhythmic ratio, i.e. the ratio of ectopic beats to the total number of beats. Hence, beta-adrenoceptor antagonism does not appear to suppress delayed ectopic activity during coronary reperfusion in conscious dogs.