1. Oxygen breath tests were performed in nine patients with diffuse pulmonary infiltration whose resting arterial Po2 (Pa,o2) ranged from 8·9 kPa to 11·8 kPa. The inspired air was suddenly replaced with oxygen for 30 s and the percentage fall in ventilation over the last 10 s was measured.
2. Pa,o2 rose rapidly and exceeded 16 kPa within 20 s in each patient.
3. The ventilation fell significantly in seven of the nine patients. It is concluded that these seven patients had a demonstrable reflex hypoxic respiratory drive at rest. This tended to be greatest in patients with the lowest Pa,o2. The percentage falls in ventilation observed were similar to those previously reported at comparable Pa,o2 levels in patients with chronic bronchitis.
4. The resting arterial Pco2 (Pa,co2) ranged from 50 to 5·8 kPa. It did not change by more than 0·3 kPa during the oxygen breath tests in any patient.