Atrioventricuiar (AV) ablation to yield chronic first degree block was attempted in six mongrel dogs. Radio frequency energy (RFE) at 750 KHz, ranging from 3.4 to 4.5 W, was delivered from the distal pole of a USCI 6F quadripolar electrode catheter to a site near the AV node. RFE delivery times ranged from 5 to 60 sec and were repeated in each animal tintil the PR interval was prolonged by 25% or more, if possible. Immediately after ablation, during autouomic blockade, 5 of the 6 dogs had PR‐interval prolongation of 25% or more, averaging 36%. The mean PR prolongation in 4 of these dogs was +41% at 1 week, +50% at 1 month ami +50% at 4 months after ablation in the absence of autonomic blocking agents and +54%, +42%, and +44%, respectively, for the same periods after autonomic blockade. The fifth dog had less than 20% increase in PR during subsequent measurements and the sixth dog experienced permanent third degree block after multiple KFE shocks. In the 4 dogs with chronic PR prolongation, the maximum damage was found in the approaches to and in the AV node, less in the His bundle, and least in the bundle branches. It is concluded that chronic modification of AV conduction can be produced in a majority of dogs with RFE with lesions located in the approaches to the AV node and at the AV node, with minimal or moderate involvement of surrounding tissue