Although much information is known about the factors controlling cerebral blood flow in the adult, there are significant physiological differences between the neonate, infant, and the adult. Therefore, it is not possible directly to transfer information concerning control of cerebral blood flow in the adult to the pediatric age population. A review of age-related differences in critical values of arterial oxygen tension (PaO2), arterial carbon dioxide tension (Paco2), systemic arterial pressure and cerebral perfusion pressure (CPP) confirms that it is necessary to establish critical values in neonates and infants for each of these variables, as well as for any possible sympathetic nervous system influences on cerebral blood flow.