Using a constant infusion of two indicators, T-1824 bound to albumin and tritiated water, flow and transit time were measured in the left coronary system of intact man. Indicators were infused for 6 minutes into the left coronary artery with sampling from the coronary sinus in a region that drained exclusively the inflow of the left coronary artery and from the brachial artery for recirculation. The degree of heterogeneity of myocardial perfusion could be defined by the time required for the curve to reach a plateau. A correlation coefficient of 0.966 was found between the two indicator-measured blood flows. The average myocardial hematocrit was calculated and found to be similar to the arterial. In the presence of myocardial disease, total flow of the left coronary artery was increased. When this was divided by tissue volume, the blood flow per unit volume of tissue was decreased in the presence of the idiopathic cardiomyopathy.