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1. The circulatory and respiratory responses to severe arterial hypoxia were studied in normal rabbits, `de-efferented' rabbits without functioning autonomic effectors, and atropinized animals before anaesthesia and during chloralose-urethane and sodium pentobarbitone anaesthesia. Net systemic autonomic activity and autonomic activity to the heart was assessed from a comparison of the responses of the various preparations.

2. In the normal spontaneously breathing animal each anaesthetic had a similar mode of action, and modified qualitatively the circulatory response present before anaesthesia. In the `de-efferented' animal the circulatory response was determined by the local effects of hypoxia, and was altered only quantitatively during anaesthesia.

3. In the normal unanaesthetized animal the reflex changes in autonomic activity during hypoxia consisted of a large increase in vagal efferent activity, a decrease in cardiac sympatho-adrenal activity, and an increase in total sympatho-adrenal constrictor activity.

4 In hypoxia during anaesthesia the vagal efferent activity no longer increased, but the change in sympatho-adrenal activity to heart and systemic circulation was the same as before anaesthesia in the spontaneously breathing animal. During anaesthesia with controlled ventilation systemic sympatho-adrenal activity increased further, and bradycardia again developed. The bradycardia was now due exclusively to reduction in cardiac sympathetic activity and not to an increase in vagal efferent activity.