INTRODUCTION The application of high-frequency current to the AV junctional area results in a temperature rise in the myocardium and may cause accelerated junctional rhythm (AJR). The aim of the study was to characterize heat-induced AJR in an in vitro animal model. METHODS AND RESULTS Studies were performed in isolated perfused pig and rabbit hearts. Using a small heating probe, we could induce AJR from a discrete area located in the middle of the triangle of Koch, which was smaller than the area from which RF energy application could elicit AJR. Histology showed that the heat-sensitive area was located over, or close to, the compact AV node. It did not correspond with the areas where double potentials were found or with the site(s) of earliest atrial activation during VA conduction. Microelectrode recordings revealed that AJR arose in nodal-type cells. Heat increased the slope of the phase 4 depolarization and shortened the action potential duration. Two types of AJR were observed: the first one was regular and the second one showed irregularity in the intervals. Interaction of multiple foci and the presence of conduction block between the foci and the His bundle caused the irregularity of the His-His intervals during the second type of AJR. CONCLUSION AJR observed during heat and RF application in the AV nodal area results from the effect of heat on AV nodal cells with underlying pacemaker activity. The heat-sensitive area is located over, or very close to, the compact AV node.