1. Minute ventilation, carbon dioxide output, and gas tensions, pH, lactate and pyruvate concentrations of arterial blood were measured in ten severely hypoxic bronchitic patients, both at rest and during slow level-treadmill walking, when breathing either air or 30% oxygen.

2. Hypoxaemia was aggravated by exercise, without change in the mean CO2 tension. Hypoxaemia was improved when 30% oxygen was breathed on exercise, but again there was no significant rise in the average arterial CO2 tension.

3. Arterial lactate concentrations, normal at rest in seven cases, were significantly increased in the last minute of exercise when breathing air, but were decreased by breathing oxygen on exercise in eight of nine cases. Arterial lactate/pyruvate ratios were not significantly changed by exercise during either air or oxygen breathing.

4. Both ventilation and CO2 output fell consistently when exercise was done breathing 30% oxygen, despite random allocation of the order of air and oxygen breathing, and the maintenance of a constant treadmill speed.

5. It is concluded that an intact chemoreceptor drive to ventilation mediated this fall in minute volume. The fall in CO2 output on exercise with oxygen does not arise from technical errors, changes in body CO2 stores, or the high work of breathing, and remains unexplained.

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