The correlation of histologic and electrophysiologic findings in dogs undergoing transvenous ablation of atrioventricular (AV) conduction has not been described. The creation of complete AV block in 10 dogs was attempted by delivering a direct-current shock transvenously through a standard tripolar electrode catheter. The catheter was positioned to record the largest unipolar atrial and His bundle electrograms. A 280 J shock was delivered to the recording electrode by a standard cardioversion unit. After 1 shock, all dogs were in complete AV block refractory to isoproterenol (1 to 4 μg/min) and atropine (0.5 to 2.0 mg). Four weeks later, 5 dogs remained in complete AV block, 1 had first-degree block, and 4 had resumed normal AV conduction. Each dog with complete heart block had histologic evidence of severe damage to the AV node, His bundle, or both. On gross examination, these dogs were found to have discrete scars at the base of the septal leaflet of the tricuspid valve. Of the 5 dogs that had resumption of AV conduction, only 1 had histologic evidence of significant damage to the AV conduction system. That animal manifested a marked increase in the P-R interval (100 to 210 ms). Although temporary heart block occurred in each animal, chronic interruption of AV conduction was more difficult. Catheter location, atrial and His bundle electrogram relations, and the electrode used for delivery of energy were factors determining the effectiveness of this technique. © 1983.