The clinical efficacy and complications associated with ablation of the atrioventricular (AV) conduction system by the selective infusion of ethanol into the AV node artery were prospectively assessed in 12 consecutive patients with medically refractory atrial arrhythmias. Six of the patients had previously failed to have permanent complete AV block created with direct current or radiofrequency catheter ablation. The AV node artery was cannulated with a 0.016 in. (0.041 cm) guide wire in all 12 patients. It was also possible to advance a 2.7F infusion catheter into the AV node artery in all patients. Transient AV block was induced by selective injections into the AV node artery of iced saline solution (8 patients) and of radiographic contrast agent (ioxaglate) (10 patients). The infusion of 2 ml of ethanol (96%) induced immediate complete AV block in all 10 patients who demonstrated AV block with ioxaglate. The escape rhythm exhibited a narrow QRS complex preceded by p His bundle deflection in nine patients and left bundle branch block in one patient. The immediate mean rate of the escape rhythm was 45.3 ± 13.4 beats/min. In two patients who demonstrated reflux of contrast agent into the distal right coronary artery with selective injections into the AV node artery, transient ST segment elevation developed in the inferior electrocardiographic leads with the infusion of ethanol. There was no change in the left ventricular ejection fraction from the baseline value (0.53 ± 0.12) to that measured after ablation (0.55 ± 0.11) and no patient developed wall motion abnormalities. All 10 patients developing complete AV block after ethanol infusion were discharged without AV conduction. After a mean follow-up period of 134.8 days (range 48 to 216), AV conduction returned in three patients (noted at 6 days and 4 and 6 weeks, respectively, after the procedure) who were discharged with complete heart block. Intracoronary ablation of the AV conduction system by the selective infusion of ethanol into the AV node artery can be performed with a low risk of serious complications. However, reflux of ethanol into the distal right coronary artery may occur. In addition, AV conduction may return in approximately 30% of patients who initially develop complete AV block. © 1991.