The postnatal development of the heart was morphometrically followed in growing golden hamsters to obtain basic information on the development of cardiovascular system of the animal. Correlations among the body weight, heart weight, heart width and length, atrial weight, right ventricular weight and left ventricular (inclusive of ventricular septum) weight, and thicknesses of right and left ventricular free walls were examined. A linear correlation was observed consistently between width and length and between body weight and atrial weight throughout the observation period. On the other hand, the results indicated that in the correlations seen between thicknesses of right and left ventricular free walls, body weight and heart weight, body weight and ventricular weight, and right and left ventricular weights, an alteration point existed at 40 to 50 days of age, which coincided with the period of achieving sexual maturity. The data have shown that the various morphometric elements of the heart correlated linearly with the body weight during the early life till 40-50 days of age, and after wards only those concerned with the left ventricular chamber preponderated as the body weight increased.