After ligation of the left coronary artery, porcine cardiac mitochondria were isolated by homogenizing the tissue and treating the myofibrillar pellet with nagarse. When compared with unligated controls, the ischemic myocardium showed decreases in phosphocreatine (to 41%), ATP (to 56%) and in the mitochondrial respiratory control index (to 69% and 78% as measured with glutamate and succinate respectively). No changes were found in the corresponding P/O ratios. Similar results were obtained upon separation of the mitochondria into two main fractions by a density gradient technique, though only one of these fractions showed a fall in succinate-supported respiration. The results suggest that ischemia decreases the NADH-dehydrogenase activity of cardiac mitochondria.