1. Changes in ventilation during progressive exercise were measured in eleven normal subjects. Ventilatory response to carbon dioxide at rest was measured in the same subjects using a rebreathing method.
2. The range of ventilatory response to exercise was 16·6–32·0 litres min−1 (litres CO2 min−1)−1 (mean 22·7; SD 5·35); response to O2 uptake was 17·0–43·9 litres min−1 (litres O2 min−1)−1 (mean 29·02; SD 9·07). Ventilatory response to CO2 (Sco2) ranged from 0·81 to 3·22 litre min−1 mmHg−1 (mean 1·87; SD 0·62).
3. There was a highly significant (P < 0·001) correlation between the changes in response to increasing CO2 output or O2 uptake, and Sco2.
4. The results are compatible with the suggestion that ventilation during exercise in normal subjects is directly related to their chemosensitivity to CO2, those having the highest sensitivity showing the greatest exercise ventilation.