Radiofrequency catheter ablation of the concealed atrioventricular (AV) accessory pathway was performed during entrainment of the AV reciprocating tachycardia. Right ventricular pacing at a rate 5 to 15 beats/min faster than the tachycardia rate was performed during the tachycardia, which resulted in transient entrainment. In 2 patients with a right-sided accessory pathway, constant fusion of the QRS complex was observed during entrainment, whereas in 2 with a left-sided pathway, no fusion beat was noted (concealed entrainment). Radiofrequency energy was applied to the accessory pathway while entraining the tachycardia. One to 4.5 seconds after initiation of energy delivery, ventriculoatrial conduction block occurred (i.e., the accessory pathway was ablated). This was associated with a change in the left ventricular activation sequence from orthodromic capture through the normal AV conduction system to antidromic capture through the right ventricle and with a change in the QRS morphology in patients with a right-sided accessory pathway. The left ventricular activation sequence and QRS morphology remained unchanged in patients with concealed entrainment. Radiofrequency energy was effectively delivered for 30 seconds with a stable ablation catheter position, and termination of rapid pacing resulted in sinus rhythm. Thus, radiofrequency ablation of the reentry circuit component during manifest entrainment fulfilled the third entrainment criterion defined previously. The results suggest that tachycardia entrainment can be used for continuous application of radiofrequency energy during tachycardia in selected patients, because it provides a constant ventricular rate during the procedure and thus maintains the catheter in a stable position.