Changes in ultrastructure after induced ventricular fibrillation were studied in the normothermic and nonhypertrophic canine heart using a transmural myocardial biopsy method. Comparisons were made between subendocardial and subepicardial layers by the quantitative evaluation of ultrastructural changes in mitochondria and glycogen granules. Slight abnormalities characterized by interstitial edema, mitochondrial derangement, contraction bands, swelling of capillary endothelium were demonstrated throughout the myocardium following ventricular fibrillation. The myocardium following ventricular fibrillation did not demonstrate differences in ultrastructure between the subendocardial and subepicardial layers. These pathologic fine structural changes were at least in part reversible. Quantification of ultrastructure showed that mitochondria and glycogen granules were well preserved. It was suggested that the myocardium following ventricular fibrillation may be less affected by the no-reflow phenomenon after restoring normal coronary circulation.