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1. From previous studies on foetal thyroid function in various mammalian species it would appear that the foetal pituitary—thyroid axis is capable of function before birth. Few studies are available to indicate the level of function before birth and in the immediate post-natal period.

2. Plasma thyroxine levels, thyroxine pool and utilization rate of thyroxine have been followed in new-born Welsh mountain lambs from birth to 39 days of age.

3. Plasma thyroxine levels are high within the first hour of life after birth (18·2 ± 2·0 µg/100 ml.) and fall steadily to a minimum of 5·1 ± 1·1 on day 11 and 12 of post-natal life. Thereafter they fluctuate about a mean of 6·1 µg/100 ml. The significance of these changes is discussed in respect to foetal thyroxine levels, placental permeability to thyroxine, and foetal and neonatal thyroid and pituitary function.

4. Thyroxine utilization rate is high both in absolute terms and relative to unit body weight on days 1-4 of post-natal life. The utilization rate/kg remains fairly constant from days 10-39 of post-natal life.

5. Thyroxine utilization in the new-born lamb is compared with that in the new-born calf in which the post-natal growth curve is very different.

6. The various factors such as levels of free thyroxine, diet and environmental changes which influence thyroxine utilization are discussed and the rate of utilization is compared with other indices of thyroid function in new-born and adult animals of different species.