The development of the metabolic response to thyroxine in the new-born pig.

1. Rectal temperature and the rate of oxygen consumption in the new-born pig were measured following the subcutaneous injection of L-thyroxine.2. Thyroxine stimulated oxygen consumption during the post-natal period; both immediate and latent increases were observed.3. The immediate effect occurred after thyroxine, 80 mug/kg, in suckled pigs aged 1-11 days; it was most marked at about 1-4 days of age with a peak increase (20%) in oxygen consumption and a rise in rectal temperature at about 4 hr following the injection.4. The peak increase (30%) in oxygen consumption associated with the latent effect of three successive daily injections of thyroxine, 300 mug/kg, occurred at 5-7 days of age, whether the pigs were first injected at 1 or at 4 days of age.5. It is concluded that there is a high sensitivity to thyroxine in the pig during the first week after birth, when the hormone is not bound to plasma proteins to the same degree as in the older animal.