Total DNA content of the whole heart muscle of rats was chemically determined, and the DNA content per single nucleus of the heart muscle cell and of the interstitial cell was determined according to the microspectrophotometric method, in order to obtain further evidence supporting the view of cell division of the heart muscle. Total DNA content of the whole heart muscle was lowest immediately after birth and increased lineally with growth and amounted up to approximately 10 times in 2 years. The DNA content per nucleus of the muscle cell and of interstitial cell was found to be constant throughout cardiac growth. These results seem to substantiate our view that heart muscle cells as well as interstitial cells divide and increase in number with the growth of the heart after birth.