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 of 3 Hthymidine (1 /Ac/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 (11^19/1000 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. KEY WORDS cardiac muscle nuclei radioautography 3 H-thymidine quantitative histology cardiac nonmuscle nuclei• It is well known that cardiomegaly may develop during postnatal life if the heart is subjected to an increased afterload. However, some controversy still exists concerning the extent to which cardiac cell hypertrophy or hyperplasia occurs in conditions leading to the increase in cardiac mass.A number of investigators induced cardiomegaly in adult animals by aortic banding or by producing aortic insufficiency and observed in the enlarged hearts an increase in total DNA but a slight decrease in its concentration (1-3). These findings would indicate both hypertrophy and hyperplasia of cardiac cells.From the Department of Physiology, University of Ottawa, Ottawa, Ontario, Canada.This work was supported by the Ontario Heart Foundation and M.R.C. of Canada.Requests for reprints should be sent to Dr. B. Korecky.Received August 16, 1971. Accepted for publication November 16, 1971. Radioautography demonstrated that the cells undergoing hyperplasia are almost exclusively nonmuscle cells (4, 5). A different result was obtained in our previous experiments in which cardiomegaly was caused by iron deficiency anemia (6, 7). We subjected rats to a diet low in iron after weaning (21 days) and observed a roughly twofold increase in heart weight at about 100 days of age. Despite this increase in mass, the number of myocardial fibers per unit cross-sectional area remained normal (6) and the concentration of DNA did not differ from the control values (7). These results, which are supported by those of Widdowson and McCance (8), suggest hyperplasia of cardiac muscle cells if cardiomegaly is induced by anemia shortly after weaning. The differences in findings between o...