SUMMARY1. To characterize the anti‐arrhythmic properties of a new calcium antagonist, monotepil, AJ‐2615, the preventive effects of AJ‐2615 were compared with those of the existing calcium antagonists, diltiazem and verapamil, in experimental models of arrhythmia.2. AJ‐2615 (0.1–3.0 mg/kg, i.v.) suppressed ventricular arrhythmias induced by adrenaline (10 μg/kg, i.v.) in rats. AJ‐2615 (0.1 mg/kg per min for 2 min, i.v.) also suppressed atrial tachycardia induced by aconitine (0.01% aconitine solution) in rats.3. In these activities, AJ‐2615 was comparable to or more potent than diltiazem and verapamil which are widely used for the treatment of arrhythmia.4. In pro‐arrhythmic activity, AJ‐2615 was less potent than diltiazem and verapamil.5. These results suggest that AJ‐2615 would be a safer anti‐arrhythmic agent, with less proarrhythmic liability than d