Publisher Summary The majority of mammals are born with a functionally, biochemically, and structurally immature central nervous system (CNS). The CNS matures gradually in the course of postnatal life, and its development may last for days, weeks, months, and even years. The rate of maturation depends mostly on the place of the given species in the phylogenetic line. Phylogenetically older parts of the CNS mature earlier than the phylogenetically younger ones. As the growth of the CNS proceeds, complicated homeostatic mechanisms develop, which in mammals are primarily of reflex and neurohumoral character. The stability of homeostasis depends to a great extent on the degree of both the phylogenetic and ontogenetic development of the CNS. Consequently, the homeostasis in the newborn mammals and in those in the perinatal period with yet immature CNS is imperfect. In these young animals, change in the environment causes significant changes in the inner milieu. On the other hand, the reaction of immature tissues to the changes of the inner milieu differs from that of the highly specialized, structurally and biochemically differentiated tissues in mature organisms. For a number of years, the reactions of organisms and primarily of their CNS to the changes of inner environment during post-natal life were being studied. Attention is focused mainly on the functional, biochemical, and structural changes of the CNS due to stagnant hypoxia and anoxia during the post-natal life of rats. Stagnant hypoxia or oligemia of the brain was brought about by the ligation of both common carotids, the circulation through the vertebrals being maintained. Stagnant anoxia or ischemia of the CNS was produced by the arrest of circulation following radial acceleration of 10g, or decapitation of the animals.