1. In contrast to the diminished ventilatory response to hypoxia which has been found at rest in cyanotic congenital heart disease, hyperventilation was noted on exercise in children who were cyanosed.
2. Sixteen children had low arterial oxygen saturations on exercise and thirteen of these hyperventilated by an amount similar to that reported in normal adults breathing hypoxic gas mixtures.
3. The three children who had little ventilatory response in relation to the increase of hypoxia during exercise all had a triad of long-standing cyanosis starting early in life, high haemoglobin concentration and low arterial oxygen saturation at rest in air.