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1. Tidal volume, carotid artery oxygen tension (P_a,O2) and blood pressure and chemoreceptor activity in the sinus nerve have been continuously measured and recorded in nine lambs anaesthetized with pentobarbitone sodium, and varying in age from some minutes after birth to 5 days after birth.

2. Inhalation of 100% oxygen caused, after a delay of 3-4 sec, a rise in P_a,O2, a fall in minute ventilation () and chemoreceptor activity. The respiratory response was abolished after section of both sinus nerves.

3. Inhalation of 10% oxygen in nitrogen caused a fall in carotid P_a,O2, a rise in respiration and in chemoreceptor activity. The respiratory response was abolished after both sinus nerves had been cut.

4. Minute ventilation, carotid P_a,O2 and chemoreceptor activity increased on breathing 5% CO₂ in air. Section of both sinus nerves did not affect the maximum increase in ventilation but the lag of the respiratory response approximately doubled while respiration increased more slowly.

5. From these results, it was calculated that the chemoreceptors had a latency of 0·25-0·5 sec and the time constant of the rate of change of chemoreceptor activity was 10-15 sec.

6. The chemoreceptors responded to changes in P_a,O2 of ±5-10 mm Hg.

7. Comparison of these results with those reported in adult animals suggest that the peripheral chemoreceptors are fully mature at birth, that their response does not differ with the age of the lamb and that the carotid body chemoreceptors are concerned both in the mediation of the hypoxic drive to ventilation and in the respiratory response to inhaled CO₂.

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