Daily urinary output of epinephrine an[ill] norepinephrine was determined on 20 normal children and on 7 children with idiopathic hypoglycemia. In normal children, the younger the child, the lower is the output. However, around puberty, a greater increase in epinephrine excretion takes place and excretion reaches adult levels by 13 to 15 years of age. Six of the seven children with idiopathic hypoglycemia showed a subnormal urinary output of epinephrine. This seems to uniquely relate to the earlier work of Hökfelt who showed that in man the fetal adrenal gland contains very little or no epinephrine, but following birth there is a gradual increase. Therefore, the pathogenesis of one type of idiopathic hypoglycemia in children and neonates could be related to a delayed maturation of the adrenal medulla and, because of this, a decrease in the adrenal gland's ability to synthesize epinephrine.