1. Ten patients with chronic hypercapnic respiratory failure (group 1) and eight patients with asthma (group 2) breathed pure O2 from an MC mask for 60 min. Blood gases were measured during this period and for the subsequent 45 min.
2. In nine of ten patients in group 1 and in all eight patients in group 2 arterial O2 tension (Pa,o2) fell to values lower than had been obtained before O2 was given.
3. These undershoots in Pa,o2 are unrelated to changing CO2 stores or to hypoventilation, and are more likely due to persistence of altered ventilation-perfusion ratios associated with O2 breathing.
4. Magnitude of the undershoots is usually small, and periods of less than 15 min off O2 are unlikely to be harmful.