Radiocardiography, a novel radioisotope method for the problemless determination of many cardiodynamic parameters which can be applied also at given physical exercise is presented. On the basis of stroke volume and heart minute volume values from 35 athletes practising different sports and of a comparison with normal values reported in the literature, differences in the cardiac adaptation and the function of athletic hearts and so-called normal hearts are pointed out. The stroke volume of endurance-trained athletes exceeds that of untrained individuals by 30-40 ml. Under exercise the increase of the stroke volume is considerably greater in endurance athletes than in individuals practising other sports or in untrained subjects. At rest the values of the heart minute volume are almost the same in athletes and untrained individuals. Under exercise the heart minute volume of endurance athletes (40 l/min) is nearly twice that of untrained individuals (volume reserve of the athlete).