(1) Radiography of hearts into which lipiodol has been injected under a pressure equal to the normal systolic blood pressure of the animals, provides a method for measuring the diameter, and consequently the cross section of the aorta and coronary arteries. (2) The errors due to pressure, temperature and post mortem changes have been investigated and the necessary compensations have been estimated. (3) Measurements of the aortic and coronary cross sections have been made on the rat, rabbit, cat, sheep, man, horse and ox. (4) The relation between aortic cross section and body weight is similar to the relation between basal metabolism and body weight in this series of animals from rat to ox. (5) In this series of animals the aortic cross section appears to vary in such a way that the velocity of blood flow in the aorta is similar in all cases. (6) In this series of animals the coronary cross section per unit of heart weight varies roughly in the same manner as the work performed per unit of heart weight. (7) In this series of animals the coronary cross section appears to vary in such a way that the velocity of the blood flow through the coronaries is similar in all cases.