Rat ventricles, ranging from 306 to 1999 mg, were obtained from normal animals of different ages and from animals subjected to chronic aortic constriction. The concentration of DNA in the paired ventricles (right and left) was found to be closely related to the concentration of nuclei in the left ventricular papillary muscles. Muscle nuclei represented only 10 to 15% of this population of nuclei. In young animals (phase 1), the total ventricular content of muscle nuclei, nonmuscle nuclei, and of DNA were increasing with ventricular growth. In the adult rat (phase 2), the total ventricular content of DNA and of both muscle and nonmuscle nuclei remained relatively constant with ventricular growth. In the enlarged hearts (phase 3), there was a further increase in total ventricular DNA but there was no further increase in the total number of muscle nuclei. Spectrophotometric studies (Feulgen stain), showed that 88% of the muscle nuclei belonged to a single ploidy class (probably diploid). No relation could be demonstrated between the extent of nuclear polyploidy and the weight of the ventricles. It was concluded that polyploidy was not a significant factor in the increased total DNA of phase 3. The increased ventricular DNA of phase 3 was explained by the proliferation of the nonmuscle nuclei.