Studies were made in anaesthetised dog of the effects of repeated acute occlusions of a branch of the anterior descending coronary artery on ventricular refractory periods in adjacent ischaemic and non-ischaemic myocardium. Differences occurred in refractoriness between normal and ischaemic areas in the ventricle. This was greatest 2.5 min after occlusion, and on release of occlusion, ventricular refractory periods reverted to normal within 5 min. Spontaneous ventricular fibrillation was directly and significantly related to the degree of dispersion of refractoriness in a given dog immediately preceding release and following release of occlusion. Infusion of isoprenaline caused significant shortening of refactory period and increased dispersion of refractoriness during ischaemia. Studies of dispersion of refractoriness should prove valuable in assessing the efficiency of metabolic or antiarrhythmic protection against ventricular fibrillation.