The aerobic metabolism of homogenates of normal and ischemic left ventricle was compared in dogs at various time intervals (15 min. to 24 hours) after high ligation of the circumflex branch of the left coronary artery. Glucose and fructose-1,6-diphosphate were used as substrates. The injured tissue was taken from the posterior papillary muscle (PP) and the non-ischemic tissue from the anterior superior septum (LV). Both oxygen consumption and organic phosphorus (Δ organic phosphorus) were slightly decreased in PP at 15 and 30 min. after ligation. Between 30 and 60 min., there was a sharp decrease in oxygen consumption and Δ organic phosphorus falling to 40 to 50 per cent of control values at 60 min. A further decrease was observed at 180 min., and at 24 hours there was a complete loss of oxidative metabolism in the PP. A striking increase in organic phosphorus metabolism and oxygen consumption was observed in the non-ischemic portion of the myocardium 60 and 180 min. after ligation.