Autoradiographic studies on the synthesis of DNA in heart muscle cells of younger normal rats after administration of ³H-thymidine were made. The labeling index of heart muscle cells in 4 hours was 8.30% on the 2nd day of life and it gradually declined to 0.15% in 3 weeks. The labeling index of interstitial cells was 9.8% on the second day of life and it similarly declined to 1.0% in 3 weeks. These results are to be added as furthr evidence to cell division of heart muscle with postnatal growth of the heart.