The mechanism of sustained ventricular tachycardias with a focal type of activation initiated by an extrastimulus was studied in isolated rabbit heart ventricle preparations by endocardial and epicardial mapping. Sixty-four electrograms were simultaneously recorded using unipolar electrodes (32 on each side of the preparation). The electrodes were spaced 3 mm apart. Data recording and processing were computerized. Sustained (greater than 0.5 min) tachycardias of a focal type were registered in 6 of 17 experiments. In each case, the three-dimensional macro-reentry mechanisms was involved. The closed circuit was around the intramural ischemic zone resulting from impairment of coronary circulation. Endocardial and epicardial points of early activation occurred at the sites where the wave passed from one surface to another.