Effects of mild hypoxic hypoxia on cerebral energy state, as assessed by phosphocreatine (PCr)/γ-ATP and inorganic phosphate (P(i))/(P(i) + PCr) ratios and intracellular pH (pH(i)) in the human visual cortex, were studied using (31)P nuclear magnetic resonance spectroscopy (MRS) at 3 T. The working hypothesis that, during compromised O(2) availability obtained by hypoxic hypoxia, both cerebral energy state and pH(i) decline due to insufficient O(2) supply for energy metabolism was addressed. Under baseline hypoxic hypoxia, with blood O(2) saturation ranging from 0.95 to 0.83, neither the PCr/γ-ATP and P(i)/(P(i )+ PCr) ratios nor pH(i) was affected, thus, showing that cerebral energy metabolism was maintained. Contrary to the formulated hypothesis, visual stimulation during hypoxic hypoxia influenced neither the indicator ratios for energy state nor pH(i) in the occipital cortex. Taking these results, together with previous observations showing that cerebral blood flow responses are the same in size both in euoxia and in hypoxia at this depth (Mintun et al. in Proc Natl Acad Sci USA 98:6859-6864, 2001; Tuunanen et al. in J Cereb Blood Flow Metab 26:263-273, 2006a), it is concluded that O(2) delivery to the brain during mild hypoxic hypoxia meets the demand by the energy metabolism both under baseline and stimulated states.