We have limited ourselves to the deleterious effects of the absence of thyroid hormones on the development of the central nervous system and have not discussed the problems caused by hyperthyroidism. A short "critical period" during which thyroid hormones are essential for normal maturation of the CNS is evident at anatomical, biochemical, and neurophysiological levels. In the last decade we have made progress toward understanding the mechanism of action of thyroid hormones, due in part to numerous studies of the ontogenesis and distribution of the nuclear T3 receptor. These studies can indicate where the molecular events that control the growth and maturation of the brain are initiated. However, much further research in this area is needed to comprehend further the relation between thyroid hormones and brain development.