In the previous paper, the author reported that 2', 3'-di-o-nicotinoyl riboflavin-4'(5')-phosphate (DNRP), one of the riboflavin derivatives, decreased myocardial contractile force, arterial blood pressure, and heart rate. In the present study, the effects of DNRP on the insulin-ouabain induced arrhythmias and on antiarrhythmic actions of quinidine, ajmaline, and propranolol were investigated. Dogs anesthetized with pentobarbital sodium were used. The following are the results obtained. 1. An intravenous infusion of DNRP (0.5 mg/min, total dose=21.0〜22.5 mg/kg) did not improve the insulin-ouabain induced arrhythmias. 2. DNRP (7.5 mg/kg, i.v.) enhanced the antiarrhythmic effect of ajmaline against the insulin-ouabain induced arrhythmias. 3. DNRP (10 mg/kg, i.v.) enhanced the antiarrhythmic effect of quinidine against the insulin-ouabain induced arrhythmias. 4. DNRP (10 mg/kg, i.v.) decreased the antiarrhythmic effect of propranolol against the insulin-ouabain induced arrhythmias.