1. Studies have been made of the response to running on a treadmill in forty boys and girls aged 5–15 years.
2. Each child was studied at two consecutive levels of steady-state work representing about 50% and 80% of his physical working capacity. Cardiac output was measured by the Indirect (CO2) Fick method.
3. The cardiac and ventilatory responses to exercise were similar to those seen during cycling on an ergometer except that the heart rate and the respiratory exchange ratio were a little lower. Stroke volume was the same.
4. The responses of children to running and cycling are discussed and the similarity to the responses of adults is noted.