Since ventricular fibrillation is the predominate cause of death following experimental coronary occlusion, measurement of the ventricular fibrillation threshold is recommended to assess the value of cardiac revascularization procedures. This determination is quantitative, precise and permits measurement under controlled circumstances. It was dissatisfaction with measurement of mortality rate following coronary occlusion in control and experimental animals (bioassay technic) that led to the adoption of the present more exact method.Acute ligation of the anterior descending branch of the left coronary artery causes an immediate drop in the fibrillation threshold to one-fifth of normal. This is not changed if one releases the artery and retests the animal with occlusion 5 to 6 months later. Implantation of the internal mammary artery into the myocardium of the left ventricle creates a greater resistance to ventricular fibrillation after coronary occlusion. In animals in which the internal mammary artery remained patent, the fibrillation threshold remained after coronary occlusion at 85 per cent (mean) of control without coronary occlusion. This was highly significant. If the internal mammary artery was thrombosed, as demonstrated by vinyl plastic injection technics, the fibrillation threshold dropped markedly after coronary occlusion and did not vary significantly from the controls. When the anterior descending branch of the left coronary artery was permanently ligated in 1 group of dogs, the expected decline of fibrillation threshold occurred. When the survivals from this group were retested 5 to 6 months later, the fibrillation threshold had increased slightly (not statistically significant) but remained below 50 per cent of control without coronary occlusion and significantly different from the threshold of animals with patent internal mammary artery implants.The results of this investigation suggest that a greater resistance to ventricular fibrillation associated with coronary occlusion can be achieved by operative means than by either mild or severe ischemia alone.