In nine patients with recurrent disabling supraventricular arrhythmia refractory to, or intolerant of, multiple drug combinations, and two patients with Wolff-Parkinson-White (WPW) syndrome and documented malignant atrial fibrillation, transvenous ablation of atrioventricular (AV) conduction, utilising synchronised unipolar DC shocks delivered by catheter to the AV node-His bundle or to the accessory AV pathway, was attempted. One to two 200-300 J discharges produced complete heart block in all of the nine patients, with markedly improved symptomatic status at one to ten month follow-up including the one patient with recovery of modified AV conduction. Single 100-150 J shocks ablated pre-excitation for five to fifteen minutes in the two WPW patients without subsequent modification of accessory pathway conduction. There were no complications. This simple technique has great potential and may supplant some open-heart procedures. With refinement it may be possible to slow rather than to ablate AV-His conduction, to ablate conduction via accessory AV pathways permanently and to interrupt ventricular re-entrant circuits.