Studies of the cardiovascular system were made among 26-mile marathon runners after at least five yearms of physical training. Studies made with the subjects at rest showed an electrocardiogram characterized by a vertical or semivertical electrocardiographic position of the heart in the chest, bradycardia, high voltage of the QRS complexes and high T and U waves. T waves which were low with respect to the height of the R waves were not encountered. Etiologically the heart was enlarged. Immediately after running 26 miles the transverse diameter of the heart was decreased considerably.