The effects of lipemia upon the coronary circulation and myocardial oxygenation, and arterial oxygen and carbon dioxide contents were studied in 13 control dogs and 20 animals with myocardial scars. Acutely induced lipemia caused a significant decrease in the cardiac output, coronary blood flow and myocardial oxygen consumption in both groups of animals. These changes were similar to those observed in experimental coronary artery embolization. The slow intravenous fat infusion did not cause a significant drop in the mean arterial pressures of these dogs. All animals in this series developed hyperpnea during fat infusion. There was a simultaneous lowering of the arterial oxygen saturation and carbon dioxide content. These changes were in some ways similar to those observed by other investigators in experimental pulmonary embolization. However, the slow fat infusion did not produce a significant elevation of the pulmonary arterial pressures of these dogs. These data suggest that the chylomicra, like most macromolecular substances, may produce mechanical interference to the coronary blood flow and impose a limitation to myocardial oxygenation, while the total oxygen consumption of the animal is unaltered by lipemia.