Brain oxygen availability (aO2) in the mature and immature fetus, neonate, and adult guinea pig was measured by a polarographic technique. Because of the similarity of fetal and maternal brain aO2 levels at various atmospheric pO2, and the marked drop in fetal brain aO2 after surgical and mechanical trauma, it was concluded that under normal physiological conditions fetal blood pO2 is probably higher than usually reported. As the effects of Apresoline, Pitressin, Adrenalin, and noradrenaline were similar in the adult and neonate shortly after delivery, it was concluded that the different effects seen in the fetus in utero were not due to qualitative differences in reactivity of fetal and maternal tissues. Autonomic reflexes play an important role in the changes in brain aO2 occurring with increased intracranial pressure. Bradycardia appears early with compression of the head and late in anoxic anoxia. The normal intracranial pressure in the guinea pig was approximately 9 mm Hg and was higher than the hydrostatic pressure of the cochlea, findings which are compatible with the existence of an open channel between cerebrospinal fluid and perilymph.