Normal cerebral blood flow (CBF), critical CBF at a flat reading of the electroencephalogram (EEG), and reversibility of the flat EEG after reperfusion were investigated in a total of 59 pigs, including seven newborns (1 to 3 days of age), 38 juveniles (1 month old), and 14 adults (7 months old). The CBF was determined by the hydrogen clearance method; the EEG was recorded continuously and a power spectrum analysis was performed. Cerebral ischemia was produced by occlusion of both common carotid arteries and induction of hypotension (approximately 50 mm Hg). The flat EEG reversibility was investigated for 3 hours after reperfusion. As parameters of brain development, the neuronal density and the time at which the S-100 protein appeared in the brain were examined. Normal CBF was highest in neonatal pigs and decreased with age. The critical CBF at a flat EEG was lowest in newborn pigs and was elevated with development of the brain. Tolerance against cerebral ischemia was greatest in newborn pigs.