Transvenous electrical ablation of AV conduction was performed in 15 patients with drug-resistant supraventricular tachycardia. Eight patients had paroxysmal AV nodal reentry tachycardia, one had permanent junctional reentry tachycardia, five had recurrent atrial flutter and one paroxysmal atrial tachycardia. The intracardiac ablation was done with 150-350 J, on average twice per patient. Permanent 3 degrees AV block was achieved in ten patients, in none of whom there was a recurrence of the tachycardia. In four patients with re-established AV conduction the clinical symptoms had nonetheless improved; despite repeated use of His-bundle ablation, permanent junctional reentry tachycardia could not be controlled satisfactorily and required surgical section of the accessory pathway. Apart from septic fever in one patient there were no serious complications. Implantation of a pacemaker, however, was required in all those patients in whom a permanent 3 degrees AV block had been produced. For this reason alone, His-bundle ablation should only be used in drug-refractory supraventricular tachycardias. But in these this method presents an important advance in treatment.