Rat brain development is a complicated process. There are significant changes of histology, cytology and molecular biology in the process of fetal and postnatal brain development. The basic structure of brain has formed in the prenatal period. While the formation of connections between different parts and the improvement of the function of the brain occur after birth, during which many of the variations are bases of the nervous system diseases, so the postnatal brain development is still very important. Although the concept of brain microenvironment has been proposed more than 150 years ago, the research about the changing extracellular space (ECS) during the postnatal brain development has yet to gain significant progress. The author reviewed the anatomical and physiological characteristics of ECS in the process of postnatal development of the rat, stating the important role of ECS in the individual development, which is expected to provide reliable evidences for the explore of mechanisms and effective treatment approaches of pediatric development related nervous system diseases.