1. We have used the rebreathing method to examine the respiratory response to CO2 in five normal subjects and twelve patients with diffuse pulmonary infiltration or fibrosis. The response to CO2 was measured in terms of both ventilation and mechanical work rate.

2. The response to CO2 was, on average, reduced in the patients compared with the normals but the patients had to perform more mechanical work to achieve a given level of ventilation.

3. Six patients had an abnormally low resting mixed venous Pco2 and four of these also had an abnormally low response to CO2.

4. The pattern of breathing was identical in patients and normals.

5. The results indicate that the reduced ventilatory response represents a true loss of CO2 sensitivity and is not simply due to mechanical limitation; but the paradox in some patients of resting hyperventilation and a low level of CO2 responsiveness is unexplained.

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