The electrophysiologic effects of multiple cryosurgical lesions along the circumference of Koch's triangle were evaluated as a treatment of AV nodal reentrant tachycardia in 3 1 dogs. These animals were divided into a control group (n = 8) and a cryosurgical group (n = 23), and were studied using incremental atrial pacing and programmed atrial premature stimulation. In the cryosurgical group, significant prolongation of AV nodal conduction time, initiation of Wenckebach periodicity at a slower atrial pacing rate, and an increase in the functional refractory period of the AV node were documented postoperatively. In contrast, there were no significant differences between preoperative and postoperative AV node conduction characteristics in the control group. Four dogs in which the effective refractory period of the AV node could be measured showed an increase of 28—104 msec following cryosurgery. In 2 dogs, discontinuous conduction curves (i.e., dual AV nodal pathways) were demonstrated preoperatively. Postoperatively, a uniform conduction curve was demonstrated. In conclusion, these results strongly suggest that this new cyrosurgical method is an effective surgical procedure for the interruption of the reentrant circuit responsible for AV nodal reentry while preserving AV conduction. © 1984, The Japanese Circulation Society. All rights reserved.