Systematic quantitative investigations were performed in the coronary arteries of 102 hearts of 36 mammal species with an overall more than tenthousandfold difference of their heart weight. After postmortem coronary angiography with a filling pressure of 100 mm Hg x-rays were taken, and the widest diameters of the coronary artery stems were determined. We found a nearly linear correlation between diameter of a standardized coronary artery and virtual diameter of heart, but the increase in diameter of coronary arteries exceeded