In our previous studies a twofold cardiomegaly associated with a proportional increase in cardiac DNA was found in rats subjected to a low-iron diet after weaning. To localize the newly synthesized DNA, control and anemic rats 24−34 days of age were given three intraperitoneal injections of3H-thymidine (1 μc/g body weight; 22c/mM) at 2-hour intervals. The radioautographs were prepared from sections of the left ventricle stained with PAS-hematoxylin. Between 26 and 34 days of age, significantly larger numbers of labeled nuclei were found in both muscle (31−160/1000) and nonmuscle cells, (29−139/1000) in the hearts of anemic rats as compared to controls (11−49/1000 and 6−35/1000, respectively). When 80-day-old rats were given a low-iron diet, a cardiomegaly of approximately 50% developed. A nonsignificant increase in total cardiac DNA and a small significant increase in labeled cardiac nonmuscle nuclei (16−35/1000) was observed in the anemic groups. We conclude that hyperplasia of cardiac muscle and nonmuscle cells is responsible for the increased cardiac mass if cardiomegaly is induced during early postweaning life, while cardiomegaly induced in adult life gives rise to hypertrophy of cardiac muscle cells and hyperplasia of cardiac nonmuscle cells.