Transvenous catheter ablation of the anomalous pathway was attempted in seven patients with drug-resistant repetitive long RP tachycardias. The patients were aged 5.5-65 years (median 20 years) and had received from four to seven antiarrhythmic agents without effect. Electrophysiological studies confirmed the anomalous substrate for tachycardia in five patients but in two patients an intranodal mechanism could not be excluded. Discharges were delivered to the coronary sinus ostium or low right atrium close to the ostium using conventional defibrillators (three patients) or a modified device (four patients). Transient AV block was seen after six attempts progressing to permanent block in one patient. In no patient could enduring block in the retrograde limb of tachycardia be achieved, and tachycardia recurred in all patients in whom anterograde conduction remained intact. The failure of selective ablation techniques in long RP tachycardia may reflect the diversity of substrate anatomy or particular properties (anatomical or functional) of the pathway compared with the usual types of anomalous pathway.