THYROID activity is increased in animals exposed to cold with release of thyroid hormone1 and both the output from, and the uptake of, iodine-131 by the thyroid is augmented2,3, but the blood-level of protein-bound iodine4 and the epinephrine-potentiating effect of thyroxine5 are decreased. Work on the evolution of thermoregulation reveals that birds and baby mammals suffer from defective thermoregulation6,7 and there is increased activity of the adrenergic system on exposure to high ambient temperature8,9. In the present investigations, thyroidal activity of birds and baby rabbits has been investigated after exposing them to high ambient temperature.