In 108 dogs, experimental infarction of the left ventricular anterior wall was produced by ligation of the descending branch of left coronary artery, and the myocardium was tested for the total content, fractional composition, and metabolism of proteins as well as for the levels of nucleic acids. The most profound metabolic changes were found in the necrotic zone, where there occur an irreversible decrease in the content of soluble proteins, an accumulation of stromatic fractions, and an inhibition of fundamental reactions of nitrogen metabolism; during the period of organization of the infarct, the ribonucleic acid level steeply rises. In the intact left ventricular segment there occur changes analogous to those in the infarcted zone, but less intense; such changes combine with some alterations of rather compensatory nature, which, however, are characteristic of those myocardial segments in which the coronary circulation has not been disturbed.