Abstract Changes in deoxyribonucleic acid (DNA), ribonucleic acid (RNA), and protein contents of myocardium have recently been used in this laboratory as a technique for determining if hyperplasia or hypertrophy of cells is paramount in experimentally produced cardiomegaly in rats. In these studies cardiac enlargement was produced as the result of a progressive anemia induced by maintaining rats on an iron-deficient diet. In order to be certain that the changes in nucleic acid and protein contents of the hearts were not due to the marginal nutritional state of the rats, the study was repeated with a more nutritionally normal preparation. The iron-deficient diet was supplemented more completely with vitamins and minerals, and the development of anemia was accelerated by weekly bleeding of each rat of approximately 25 per cent of the estimated total blood volume. At 112 to 119 days of age, the rats in both experimental and control groups were killed. The hearts were analyzed by colorimetric techniques for DNA, RNA, and protein. In consonance with the previous study, the hearts of the anemic rats had a statistically significant increase in mean weight, RNA, and protein content, and RNA-to-DNA and protein-to-DNA ratios. Again, DNA concentration was significantly decreased in the group with cardiomegaly, so that total ventricular content of DNA was essentially the same in both groups of rats. The results are interpreted as suggesting that the cellular changes which occurred in the myocardium were those of hypertrophy rather than hyperplasia. It appears that nucleic acid and protein determinations are useful techniques for studying cardiac enlargement.