1. The effects of the intravenous injection of Kö 1173, a new anticonvulsant drug, phenytoin and procainamide were studied on three types of cardiac arrhythmia in dogs.2. Ventricular ectopic beats produced by intravenous injection of adrenaline in anaesthetized dogs respired with halothane were abolished by Kö 1173, 0.6 +/- 0.1 mg/kg, phenytoin, 1.1 +/- 0.3 mg/kg and procainamide, 4.1 +/- 1.8 mg/kg.3. Ventricular tachycardia was produced in anaesthetized dogs by the intravenous injection of ouabain and the three drugs infused intravenously at 0.2 (mg/kg)/min until sinus rhythm returned. Kö 1173 was effective in 8 out of 9 dogs after a mean dose of 1.3 +/- 0.3 mg/kg; phenytoin in all 3 dogs after 2.7 +/- 0.6 mg/kg and procainamide in the 3 dogs tested after 16.6 +/- 1.3 mg/kg.4. The intravenous injection of Kö 1173, 8.0 mg/kg, greatly reduced the number of ventricular ectopic beats occurring in conscious dogs 18-44 h after ligation of the anterior descending branch of the left coronary artery, with a resultant increase in the number of sinus beats. Phenytoin, 8.0 mg/kg, had a similar effect but procainamide was much less effective.5. These results indicate that Kö 1173 is effective in abolishing experimental cardiac arrhythmias and suggest that its effects should be studied in patients.