Atrioventricular block was produced by infusion of 0.1-0.2 ml of 40% formaline into the A-V node or the bundle of His. Effect of diethylamine analog of etmozin (1 mg/kg) and cesium chloride (20 mg/kg) on the idioventricular rate was studied in control and experimental dogs 24 hours after the occlusion of the coronary artery. It was shown that cesium ions inhibit the idioventricular rate in the normal canine heart (from 49.9 +/- 3.8 to 6.9 +/- 4.5/min; p less than 0.001, n = 7) and have no influence on the ectopic ventricular rate in the late stage of myocardial infarction. On the contrary, diethylamine analog of etmozin has no significant effect on normal ventricular automatism and considerably reduces the ectopic ventricular rate (from 111.5 +/- 7.2 to 23 +/- 15.6/min; p less than 0.001, n = 8). The evidence points to the difference between the ionic mechanisms determining the normal specialized conduction system automatism and the ionic mechanisms which are responsible for the enhanced automatism in the late stage of experimental myocardial infarction.