Ventricular tachycardia (VT) is a life-threatening arrhythmia. In patients with myocardial infarction, it is caused by a reentrant circuit, formed by conduction blocks and a slow conducting zone within the infarcted area. Catheter interventions consist of identifying those circuits and breaking them by radiofrequency ablation. Here a novel method is presented for identification of ablation targets. It consists of pacing the heart from various sites of the ventricle. As the catheter is moved, changes of the activation wavefront can be detected by analysis of surface electrocardiograms. Areas of abrupt changes thereby correspond to critical zones likely to sustain VT circuits.