Analysis of the T loops of the vectorcardiogram (Frank's lead) in young healthy Japanese and its changes with aging and sex distinction were done. Results were as follows. 1) The magnitude of the QRS and T maximum vector was the greatest in the first decade in the male and in the second decade in the female. 2) T magnitude/QRS magnitude Ratio was greater in the male than in the female. 3) L/W ratio was smallest in the first decade. That is the T loop in first decade was most circular. 4) The polar vector of the T loop was located in the second quadrant, and the greatest in the first decade. 5) The T loop was large and rounded ellipse In shape. On the other hand, the T loop found in coronary sclerosis and the hypertensive was small and circular. 6) It is postulated that the change of shape of the T loop was influenced with the vector of developing posterior wall of the left veutricle.