Two hundred rabbits aged from 0 to 21 days were used for studies of the functional activity and maturity of the thyroid and hypothalamo-hypophysial system in the newborn rabbit. Histological findings showed that there was a rapid increase in the thyroid weight, attributed to somatic growth. The calculated relation between thyroid (Y) and body weight (X) was found to be Y = 0·3404X0·7726. The hormonal iodine in the blood and thyroid gland:serum radioactive concentration ratio increased with age. These variations in thyroid gland activity were paralleled by maturation of the hypothalamus (as judged from the appearance of the neurosecretory material). The supraoptic nucleihypophysial system of the rabbit appeared to be morphologically mature at about 7 days after birth. A differentiation can be made between the 'functional development' of the thyroid gland and its 'basal growth', which is part of the general growth of the body.