We recorded a discrete 0.95 mV potential consistent with accessory atrioventricular pathway (AP) activation during serial electrophysiologic studies in a patient with Ebstein's anomaly and Wolff-Parkinson-White syndrome. Bipolar pacing from the catheter electrode in which the AP potential was recorded resulted in a stimulus-ventricle interval identical to the AP-ventricle interval during antegrade conduction, and a stimulus-atrium interval identical to the AP-atrium interval during retrograde conduction. With the patient in the drug-free state, antegrade AP block during atrial pacing and retrograde AP block during ventricular pacing occurred distal to the AP potential (AP-ventricle junction and AP-atrium junction, respectively), supporting the "impedance mismatch" hypothesis. Procainamide and disopyramide each lengthened the antegrade AP effective refractory period by affecting the AP-ventricle junction (possibly by decreasing the current generated by the AP). Both drugs also lengthened the retrograde AP effective refractory period but produced a greater effect on the ventricle-AP junction than on the AP-atrium junction, suggesting marginal geometry of the former. R wave synchronous shocks of 160 and 320 W-sec delivered between the catheter electrode recording the largest unipolar AP potential and a skin electrode produced transient, complete, antegrade block over the AP, suggesting the feasibility of this new nonsurgical technique for AP ablation.