The direct endocardial surgical techniques recently introduced for the treatment of refractory ischemic ventricular tachyarrhythmias have resulted in decreased surgical mortality rates and increased success rates in comparison to previous indirect techniques. Since the mechanism of action of one of these new techniques, the encircling endocardial ventriculotomy (EEV), is unknown, the present study was designed to clarify the electrophysiological effects of this procedure. Epicardial and intramural electrophysiology was studied in 18 dogs before and after undergoing an EEV. In the absence of induced myocardial ischemia, the procedure caused an epicardial conduction delay of 23 ± 3 msec (p < 0.0001) across the boundaries of the incision. When the EEV encircled acutely ischemic myocardium, it was capable in certain instances of isolating spontaneous ventricular electrical activity to the myocardium encompassed by the incision and thereby protecting the remainder of the heart from the arrhythmia. The EEV resulted in total ablation of all fast (2 Mv/msec) electrical activity at 20 of 48 (42%) subendocardial electrode sites and at 12 of 44 (27%) subepicardial sites monitored within the encompassed myocardium. These data suggest that although the EEV may be capable of isolating ischemic ventricular tachyarrhythmias to the encompassed myocardium, it most commonly ablates the anatomic-electrophysiological substrate necessary for the genesis and perpetuation of these arrhythmias.