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1. Intravenous infusions of adrenaline and noradrenaline in doses averaging 0·8 µg/kg.min increased the respiratory minute volume of anaesthetized cats breathing room air. The mean increase in respiratory minute volume was 14% during adrenaline infusion and 8% during noradrenaline infusion.

2. In a small group of decerebrate cats infusions of adrenaline and noradrenaline increased ventilation by 19 and 27% respectively.

3. Intravenous catecholamine infusions also increased the respiratory responses of anaesthetized animals to the inhalation of 5% or 10% O₂ in N₂ and to the inhalation of 5% CO₂ in air.

4. Adrenaline and noradrenaline infusions had no significant effect on the ventilation of animals breathing 100% O₂, nor did they significantly alter the respiratory response to the inhalation of 5% CO₂ in O₂.

5. After section of the carotid sinus and aortic nerves, a blood-pressure compensator being used to minimize changes in arterial pressure, catecholamines had no effect on the respiration of cats breathing air.

6. An increase in carotid body chemoreceptor discharge accompanied the increase in ventilation during catecholamine infusion.

7. Intravenous catecholamine infusions still produced an increase in ventilation and carotid body chemoreceptor discharge after both aortic nerves and both cervical sympathetic nerves had been cut.

8. Intra-arterial infusions into one carotid artery of 0·2 µg/kg.min of adrenaline or 0·1 µg/kg.min of noradrenaline led to mean increases in respiratory minute volume of 9·9 and 11·5% respectively. No increase occurred after section of the corresponding carotid sinus nerve. Such infusions also evoked an increase in carotid body chemoreceptor discharge.

9. It is concluded that the hyperpnoea produced by adrenaline and noradrenaline infusions in the cat is predominantly reflex in origin and is mediated by the arterial chemoreceptors.

10. The increase in ventilation produced by adrenaline appears to have a component additional to its effect upon the chemoreceptors though the nature of this action has not been identified.