DURING development of the central nervous system (CNS) both cellular and extracellular spaces and the permeability of the CNS to various substances undergo certain changes. For example, cytological studies have shown that both the number and the volume of glial cells increase with maturation whereas the number of neurons remains constant and only the neuronal volume increases.2,3 It has also been shown that the brain extracellular space decreases during development.7,19 Methods for studying the various physiological compartments of the CNS have been developed by Woodbury,20 Reed and Woodbury,12 and Reed et al.13 The rate of entrance of radioactive substances such as sucrose, iodide, chloride, inulin, and bromide into the CNS and their volume of distribution were used to study the various physiological compartments of the brain and the interchange of substances between these compartments. However, such studies have been confined to adult animals and no