Intact vagus nerves are necessary for consistent induction of ventricular arrhythmia by adrenaline in barbiturate-anesthetized dogs although sinus bradycardia does not necessarily precede ventricular pacemaker activity. We have found that the threshold dose of adrenaline is not affected by maintaining the atrial rate electrically. In driven hearts, arrhythmia is preceded by partial or complete atrioventricular (AV) nodal block. AV nodal block is also observed without emergence of the ventricular pacemaker. AV nodal block during "nodal rhythm" induced by adrenaline in normal hearts has been demonstrated. It is concluded that although ventricular slowing always precedes ventricular arrhythmia in these preparations, the action of the vagi on nodal conduction is sufficient to allow emergence of the ventricular pacemaker both in the presence and the absence of maintained atrial rates.