The electrophysiologic properties of a lidocaine analog (Kö 1173) was experimentally studied in 41 mongrel dogs. The threshold for occurrence of repetitive ventricular extrasystoles was significantly raised by the administration of 2, 4, and 8 mg/kg of the drug, while atrioventricular and intraventricular conduction times were not affected. After ligation of the anterior descending branch of the left coronary artery, the threshold for occurrence of repetitive ventricular extrasystoles was significantly lowered before the administration of Kö 1173 but it returned to control values after the administration 1 mg/kg of the drug. The strength-interval curve shifted progressively to the right when the dose was increased from 2 to 4 and 8 mg/kg. These results indicate that Kö 1173 prevents the decrease in threshold for occurrence of repetitive ventricular extrasystoles in acute coronary insufficiency at a dose which does not affect the conduction system.