Hypoxia increases cerebral blood flow (CBF), but it is unknown whether this increase is uniform across all brain regions. We used H₂¹⁵O PET imaging to measure absolute blood flow in 50 regions of interest across the human brain (n= 5) during normoxia and moderate hypoxia. P_CO2 was kept constant (~44mmHg) throughout the study to avoid decreases in CBF associated with the hypocapnia that normally occurs with hypoxia. Breathing was controlled by mechanical ventilation. During hypoxia (inspired PO₂ = 70 mmHg) mean PETO₂ fell to 45 mmHg ± 6.3 (s.d). Mean global CBF (gCBF) increased from normoxic levels of 0.39ml/g (± 0.13) to 0.45ml/g (± 0.13) during hypoxia. Increases in regional CBF (rCBF) were not uniform and ranged from 9.9%± 8.6 in the Occipital lobe to 28.9% ± 10.3 in the nucleus accumbens. Regions of interest (ROI) that were better perfused during normoxia generally showed a greater rCBF response. Phylogenetically older regions of the brain tended to show larger vascular responses to hypoxia than evolutionary younger regions, e.g. the putamen, brainstem, thalamus, caudate nucleus, nucleus accumbens and pallidum received greater than average increases in blood flow, while cortical regions generally received below average increases. The heterogeneous blood flow distribution during hypoxia may serve to protect regions of the brain with essential homeostatic roles. This may be relevant to conditions such as altitude, breath hold diving, obstructive sleep apnea and may have implications for functional brain imaging studies that involve hypoxia.