After the induction of ventricular fibrillation (VF) in 29 animals, ECGs were serially recorded to determine whether a very low amplitude or a straight line ECG could occur in some leads while coarse fibrillatory waves were present in the other leads. Ventricular fibrillation was electrically induced via a transvenous catheter electrode advanced into the right ventricle in the initial group of 11 animals. Five of the 11 had straight line ECGs recorded in lead aVR or aVL whereas coarse VF was present in the other frontal plane leads. In the second group of 7 animals, VF was induced by placing one needle electrode into the apex of the left ventricle and the other in the opposite chest wall. Two of the 7 had straight line recorded in either aVR or aVF. In the last 11 animals, regional ischemia followed by reperfusion was induced by ligation and release of the coronary arteries. Spontaneous VF occurred after coronary occlusion and reperfusion in 8 and was electrically induced in 3. None of these 11 animals had a straight line recorded in any of the frontal leads after the first 2 min of VF. VF was terminated at 2 min in these animals by direct current defibrillation. One of the animals refibrillated 10 min after defibrillation. This animal had serial frontal plane ECGs at 1-min intervals during the 11 additional min of VF. A straight line was recorded in aVL in each of the sets of frontal plane leads.These observations lead us to conclude that VF may not always be caused by numerous small reentry ringlets of depolarization but may occur with larger irregular waves of depolarization, producing an electrical vector. The direction of the depolarization waves may depend upon the site of initiation. The result of this vector is an electrical null plane that can produce the ECG appearance of ventricular standstill in one lead while coarse fibrillatory waves are present in others.