ECG characteristics of well-trained endurance athletes who are in state of training vagotonia were investigated in comparison with male and female controls of the same age group. The effects of sympathicotonia on these ECG characteristics of training vagotonia were also investigated by recording ECG immediately after 100 meter dash in endurance athletes. Thus, ECG charcteristics of endurance athletes could be divided into 2 groups, an cute and a chronic effect of maximal exercise. On the basis of these observations and inferences the athletic heart was clssified into 5 stages, and their probable ECG manifestations were reasoned. © 1969, The Japanese Circulation Society. All rights reserved.