1. The effect of lung deflation was studied during treatment of patients for spontaneous pneumothorax: (a) in four patients acute lung deflation was produced by opening the chest drain to the atmosphere and the immediate effect on breathing was observed and (b) breath-holding time and the ventilatory response to CO2 were determined on admission to hospital and after recovery in a further four patients.

2. Allowing one lung to deflate suddenly produced an immediate increase in respiratory rate and electrical activity in inspiratory muscles.

3. Maximum breath-holding times were always decreased by pneumothorax.

4. In the presence of pneumothorax, the ventilatory response to rebreathing CO2 was increased at the break point, the respiratory rate was increased and the Pco2 at the breakpoint decreased in three out of four patients.

5. These results are consistent with the hypothesis that lung deflation has a reflex excitatory effect on breathing (Hering—Breuer deflation reflex). If this hypothesis is correct, it would appear that the threshold for the reflex is higher than for other mammalian species.

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