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1. In the sinus nerve of mature foetal sheep (120-147 days gestational age) three types of nerve activity have been demonstrated: (a) baroreceptor afferent, in which the nerve activity was synchronous with the pulse pressure wave and which followed pressure changes in the carotid sinus including those caused by compressing the umbilical cord. The discharge of nine out of twelve baroreceptor afferents increased with sympathetic stimulation, (b) a subtype of (a) in which there was irregular activity from pressure receptors which most probably represented baroreceptors discharging at or about threshold, and (c) in the older foetuses (130-147 days), there was activity which was unaffected by changes in carotid sinus pressure, P_{a, O2} or NaCN but which increased in discharge rate with compression of the umbilical cord or with stimulation of the cervical sympathetic; this activity then increased with injection of NaCN and was thus identified as chemoreceptor in origin. In addition, a pathway originating in the cervical sympathetic and which passed through the superior cervical ganglion was demonstrated by evoking a compound action potential on stimulation of the preganglionic cervical sympathetic nerve.

2. Recordings from pre- or post-ganglionic cervical sympathetic nerves showed that spontaneous activity was present and increased when the umbilical cord was compressed.

3. These results suggest that the carotid body chemoreceptors in the foetus are relatively insensitive to chemical changes and that, in their activation at birth, the sympathetic could be involved. Possible neural pathways and the mechanism of activation are discussed.