Very small doses (0.1 to 2.0 μg./Kg.) of epinephrine injected in 1 minute into dogs anesthetized with 20 per cent cyclopropane cause a coupled rhythm which is usually bigeminal and characterized by an exceptionally constant interval between the coupled beats. Phenyiephvine also causes this arrhythmia, methoxamine does so rarely, but isoproterenol does not. An elevation in systolic pressure and possibly a tachycardia are required for the appearance of bigeminy. However, this arrhythmia can be elicited consistently by these factors only in the presence of a sympathomimetic amine with cardiac stimulant actions. A focus of automaticity has been ruled out in the genesis of the abnormal ventricular beat by demonstrating the constancy of the coupling interval despite sudden changes in the atrial rate. Dichloroisoproterenol appears to attenuate the induction of bigeminy by epinephrine to a considerably lesser extent than the major arrhythmias expected on injection of large doses of this sympathomimetic.