Catheter ablation of the atrioventricular node (AVN) with radiofrequency current is closely associated with the short-term onset of a junctional rhythm. The origin of this rhythm was analyzed in Beagle dogs which were anesthetized with pentobarbital sodium. Atrioventricular (AV) conduction block was induced first using a standard catheter ablation technique for the AVN, so that the sinus automaticity could not override the junctional ectopy during the following energy delivery. The ablation catheter was kept in the initial position and the delivery of radiofrequency energy was repeated. The pattern of ECG changes suggests that the dominant pacemaker may shift from the distal portion of the AV junctional area to the proximal portion during the energy delivery. This enhanced junctional automaticity was suppressed by the β-blocker esmolol, but was not affected by M-antagonist atropine. Moreover, the β-agonist isoproterenol did not induce the same type of junctional tachycardia, but the pacemaker shift was induced by the increased sympathetic tone after transient asystole by ventricular overdrive pacing or acetylcholine administration. These results suggest that proximal portion of the AV junctional area has extremely slow pacemaker activity, but responds to locally released norepinephrine with an abrupt rise and fall in rate, resulting in a typical pattern of junctional tachycardia during the ablation of the AVN. (Circ J 2002; 66: 696 - 701)