Types of afferent nervous activity which may be measured in the vagus nerve of the sheep foetus.

1. Afferent activity was measured in the vagus nerves in sixteen foetuses within 2-3 days of natural term and in two foetuses 130 and 135 days gestational age, exteriorized by hysterotomy from seventeen ewes given either a spinal anaesthetic (ten) or pentobarbitone sodium (seven).2. Activity from aortic chemoreceptors was observed which increased in rate with foetal hypoxia, NaCN or nicotine and temporary occlusion of the umbilical cord or veins. Activity was abolished or reduced with saline equilibrated with air.3. Activity from baroreceptors was observed which was synchronous with the arterial pulse and which followed changes in arterial pressure.4. Activity designated as arising from stretch receptors in the lungs or lower airways was observed which consisted of a regular discharge at 15-45 impulses sec(-1), which varied with intra-tracheal pressure, which increased with negative intrathoracic pressure generated by spontaneous respiratory movements, which, when the umbilical cord was occluded, at first increased in rate and then fell to zero at or about the occasion of the first breath. After breathing had been established, activity assumed the rhythmical pattern and showed evidence of slow adaptation of receptors seen in the adult. This activity was frequently modulated by the heart beat and after breathing had started it was reduced when carbon dioxide was raised.5. Respiratory movements with a frequency of 10-20/min occurred spontaneously in foetuses from ewes given a spinal anaesthetic and were associated with a modulation of the stretch receptor discharge. They could be induced by withdrawing fluid from the trachea and so lowering both intra-tracheal pressure and receptor discharge and were abolished by raising intra-tracheal pressure. These changes were abolished by sectioning the vagi just caudal to the nodose ganglion.6. These results indicate that there are excitatory and inhibitory pathways in the vagus nerves which are active in the foetus and which may well be concerned with the limitation of respiratory movements of the foetus and the onset of maintained respiration at birth.