In thirty-seven children free of neurological symptoms, we attempted the standardization of hyperventilation on EEG. We also attempted to determine the quantity of hyperventilation activation necessary to produce equivalent degrees of EEG slowing at different ages. The respiratory rate (RR), total expiratory volume/min (VE), O2 consumption volume/min (VO2), expiratory CO2 volume/min (VCO2), tcpO2 and tcpCO2 were monitored before, during and after hyperventilation. The optimal conditions for adequate activation were found to be: a respiratory rate of 30/min, a 3-fold elevation of VE and a duration of 4 minutes. With this activation, the degree of EEG slowing was found to be nearly inversely proportional to the age (in the age range of 6 years to 17 years old). Therefore, this activation may be adequate and useful for evaluating the EEG development in childhood. As to the relationship between the appearance of EEG slowing and changes in respiratory factors, the pCO2 decrease and the cerebral blood flow decrease, which may be evoked by the pCO2 decrease, are the most fundamental factors that produce EEG slowing during hyperventilation. The difference in the response on hyperventilation between children and adults may be due to age-related CNS sensitivity to CO2 and/or cerebral vascular CO2 responsiveness.