The Frank-Starling mechanism was investigated in 88 athletes exposed to physical stress and was shown to operate under physical stress in athletes with ventricular cavities of normal or moderately increased size. In athletes with physiological ventricular dilatation as a result of endurance training (over 160 ml in ultimate diastolic volume), the Frank-Starling mechanism is not normally triggered under stress: increased cardiac output is provided by greater basal blood volume reserve. With ultimate diastolic volumes of 115-159 ml, the Frank-Starling mechanism provides an optimum increase in peak stroke volume. The effectiveness of the heterometric mechanism activated by physical stress in subjects with small ultimate diastolic volumes is not sufficient as additive reserve volume cannot be increased essentially.