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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 mine 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 of O2 are unlikely to be harmful.

between smoking and blood sampling. At the time of sampling a detailed smoking history was taken. In cigar and pipe smokers particular care was taken to differentiate between primary cigar and pipe smokers and those who had previously been regular cigarette smokers. Primary pipe and cigar smokers were defined as subjects who had smoked less than one cigarette a day for one year.Samples were stored at 4°C and were analysed in batches. Measurements were made in duplicate with an IL182 CO Oximeter, which was fully calibrated before each batch of measurements. 1 2 Results Table I shows the distribution of carboxyhaemoglobin concentrations. The lowest concentrations were seen in the primary cigar smokers (mean 0.93% of total haemoglobin, range 0.3-2.2%) and primary pipe smokers (mean 1.36%, range 0.8-1.6%). The highest carboxyhaemoglobin concentrations were seen in secondary cigar smokers (mean 6.8%, range 0.9-14.7%). The secondary pipe smokers had rather lower concentrations (mean 3.39%, range 0.5-8.3%). The highest individual concentrations were found in cigarette smokers (mean 4.8%, range 0.4-16.7%).