Transvenous ablation of atrioventricular conduction by high energy shock through a pacing wire has been used as a method of controlling the ventricular response to rapid atrial arrhythmias. A patient in whom this technique successfully controlled refractory tachycardia was admitted with severe left ventricular impairment five months after the procedure and died. The heart was examined at necropsy. There was no sign of endocardial damage in the region of the atrioventricular node-His bundle. Histological sections failed to show any scarring, inflammation, or infiltration in the region of the atrioventricular node. On the contrary, there was no evidence of any surviving atrioventricular nodal tissue. The mechanism of this effect is not known. The lack of endocardial or myocardial damage further confirms the safety of this new method of controlling tachycardia.