A hypothesis of a principle of the adaptation of heart size to volume load was generalized from some experimental findings in literature on hearts of a few mammalian species. The hypothesis was that the heart adapts its size and weight so that both myocardial force per unit cross sectional area and extent of shortening per unit length be kept constant. From these 2 conditions, mathematical relationships of heart weight and enddiastolic volume to stroke volume were derived by the aid of a thick wall sphere model of the left ventricle. The calculated result was in good agreement with comparative physiology data in literature on normal hearts from rats to cows. The present hypothesis was therefore suggested to be a basic principle of heart adaptation to the natural volume load in a wide variety of mammalian species.