The relative amount of myocardium perfused by the three large coronary arteries was determined in 171 human hearts postmortem. Roentgenograms of transverse serial sections of the ventricular myocardium enabled planimetrical measurements. With little variation, an average of 41.5 per cent of the entire ventricular myocardium was supplied by the left descending coronary artery. Both left branches supplied an average of 63.8 per cent and the right coronary artery supplied 36.2 per cent of the myocardium. The size of supplying areas, in particular that of the right coronary artery and the left circumflex branch, was mainly dependent upon the coronary artery types. As a rule cardiac hypertrophy did not influence the size of coronary supplying areas as much as did the coronary artery types. Only very few hearts revealed that the myocardium was supplied to a greater extent by the right coronary artery than by the left (5.3 per cent). There is a close relationship between the size of the myocardial supplying area and the lumen of the corresponding coronary artery.