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1. Methods are described for measuring tidal volume and frequency, end-tidal CO₂, blood pressure and heart rate, and arterial gas tensions in the unanaesthetized new-born lamb.

2. The resting values of minute ventilation ()/kg body wt. and arterial oxygen and carbon dioxide tension, (P_{a, o2}) and (P_{a, CO2}) were similar to those which have been reported in the new-born baby. There was a direct and significant relation between P_{a, o2} and P_{a, CO2} and the age of the lamb.

3. Thirty-five unanaesthetized lambs aged 40 min to 10 days breathed 100% oxygen; minute ventilation fell by an average of 19% of control, end-tidal CO₂ increased and the ratio of change in tidal volume (V_T) to change in pressure (P) (V_T/P) remained constant. In a proportion of lambs, a small decrease in blood pressure and heart rate was observed. The effect of breathing 100% O₂ on lung compliance was variable.

4. These changes in ventilation were virtually abolished after both sinus nerves had been cut.

5. The results therefore suggest that a significant hypoxic drive to ventilation exists in the new-born lamb and that this drive is mediated by functioning and mature peripheral chemoreceptors.

6. Preliminary evidence suggested that, on 100% O₂, the sensitivity of new-born lambs to inhaled CO₂ was reduced.