By catheterizing the coronary sinus it is possible to make chemical comparisons of the venous with the arterial blood in the coronary vessels of the human heart in situ. Metabolic studies by this technique have demonstrated the importance of fatty acids in the nutrition of the myocardium. By the introduction of microelectrodes into the interior of single cells from the myocardium, the atrioventricular node, and the ventricular conducting system it is possible to follow electric changes and ionic fluxes in small units. This method has thrown new light on the significance of potassium and oxygen and the mechanism of fibrillation and pulsus alternans. Comparable advances have been made in the chemistry of myocardial proteins, the basic pharmacology of the heart, the pathological physiology of ischemia and anoxia, and the metabolism of the myocardium in congestive failure or after infarction.