1. Maximal oxygen uptake (V̇o2 max.), lung volumes, and ventilatory responses to carbon dioxide and hypoxia have been measured in identical twin athletes, who were trained to a similar high degree.
2. The results confirm previous findings for V̇o2 max. and lung volumes in identical twins, and are in keeping with the suggestion that genetic factors play a major part in determining the ventilatory response to carbon dioxide and hypoxia.