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1. In anaesthetized cats, the efferent discharge recorded from slips of otherwise intact sinus nerves was sparse in eupnoeic conditions but increased markedly during systemic hypoxia or asphyxia or following the injection of cyanide or acetaldehyde into the circulation of the ipsilateral carotid body.

2. When the sinus nerve was cut distal to the efferent slip the responses to cyanide or acetaldehyde were abolished. The sparse `resting' activity which remained was increased following the intravenous injection of adrenaline. Following distal section, the impulse traffic of the efferent slip did increase during systemic hypoxia but the response was much feebler than when the nerve was otherwise intact.

3. The impulse activity of most efferent slips, peeled off from the otherwise intact sinus nerve, was abolished when the glossopharyngeal nerve was cut central to its junction with the same sinus nerve, indicating that the activity was probably recorded from genuine efferent units. The discharge of some `efferent' preparations was still present, however, following such section and showed an increase to local injections of cyanide. This activity was probably recorded from looping or branching chemoreceptor afferents.

4. The discharge of efferent slips of the cut aortic nerve was increased following the intravenous injection of adrenaline and during systemic hypoxia. These responses were not present when the vagus nerve was cut central to the nodose ganglion.

5. In eupnoea, chemoreceptor afferent activity recorded from slips of the sinus or aortic nerves is much the same whether these nerves be otherwise intact or whether they be cut. During systemic hypoxia, chemoreceptor afferent discharge was less when it was recorded from the otherwise intact nerves than when these nerve trunks were cut.

6. The cell bodies of sinus nerve efferent fibres are synaptically excited by chemoreceptor afferents coursing in the same nerve trunk. The increase of efferent impulse activity aroused by this means depresses chemoreceptor afferent discharge.