The present study describes a surgical method for the permanent prolongation of atrioventricular (AV) conduction. In 22 dogs, a series of nine discrete cryolesions were placed along the perimeter of the triangle of Koch while the His bundle electrogram was being monitored continuously. Ten dogs underwent a sham operation to control for the effects of cardiopulmonary bypass and atriotomy on AV conduction. The conduction time through the AV nodal region was significantly (p less than 0.01) prolonged acutely (3 hours postoperatively) and chronically (14 weeks postoperatively) in the animals subjected to cryosurgery. Thus selective cryosurgery in the AV nodal region produces a permanent alteration in AV conduction. This cryosurgical technique may result in ablation of the electrophysiological substrate required for paroxysmal supraventricular tachycardia resulting from AV nodal reentry.