BACKGROUND: Second degree AV block is occasionally induced during AV nodal reentrant tachycardia by programmed electrical stimulation. This study was performed to determine the incidence, the block site, and the mechanism of AV block during AV nodal reentrant tachycardia. METHODS AND RESULTS: The study population was 67 consecutive patients with AV nodal reentrant tachycardia studied by programmed electrical stimulation. Among these patients, common types(slow pathway for anterograde and fast pathway for retrograde conduction) were 64 patients and uncommon types(fast pathway for anterograde and slow pathway for retrograde conduction) were 3 patients. Among 67 patients with AV nodal reentrant tachycardia 10 patients developed 2:1 AV block during tachycardia. The block site of AV block was infrai bundle in 9 patients and supraHis bundle in patient. There were intermittent aberrancies of RBBB and/or LBBB form during tachycardia in 5 patients with infraHis AV block. CONCLUSION: The occurrence of AV block during AV nodal reentrant tachycardia is not rare and might be related to the prematurity of atrial extrastimulation. The block sites of AV block were infraHis bundle in most cases and this finding suggests that distal common pathway of the reentry circuit is present in the AV node.