The development of epinephrine, norephinephrine, and total catecholamine secretion in plasma and andrenal glands was studied in newborn rats at short intervals: at day 2, 4, 6, 8, 10, 12 and 23. The increase in the plasma level of epinephrine represents a maturation of the secretion of the adrenal medulla. The increase in plasma of epinephrine and norepinephrine and the content of catecholamines in the adrenal glands of both normal animals and those treated with either high doses of T4 or cortisol at birth suggest a slowing down of the normal development of epinephrine secretion. This was confirmed by inducing hypoglycemia in these three groups of animals by a 20-h fast or by insulin administration (0.1436 mumol/kg). We conclude that both high doses of T4 and cortisol administered at birth seem to retard the development of the autonomic nervous system similar to the effect on the central nervous system.