Acute hypoxia is known to increase cerebral blood flow and intracranial pressure in the fetus and newborn. The combined effects of these changes on cerebral perfusion pressure are unknown. We investigated cerebral perfusion pressure changes in newborn puppies of two age groups (group I, mean age 28 days, group II, mean age 11 days) during acute, severe hypoxia. The results indicate that (1) in both groups the intracranial pressure increases within seconds following the onset of hypoxia; (2) the mean arterial blood pressure drops almost precipitantly in older animals, whereas in group II, the blood pressure is maintained near baseline values up to 4 min, and (3) because of differences in blood pressure responses the cerebral perfusion pressure falls dramatically in group I, whereas in younger animals normotensive response to hypoxia helps maintain the cerebral perfusion pressure for a longer period. We conclude that in young animals early normotensive response to hypoxia assures adequate transmural pressure for the perfusion of brain which may be a built-in protective factor.