We measured cerebral phosphocreatine (PCr), inorganic phosphate (Pi), ATP, and intracellular pH (pHi) with in vivo phosphorus nuclear magnetic resonance (NMR) during 10- to 15-min periods of reversible hypoxic hypoxia in 20 newborn lambs (1-11 days). There was a significant correlation between arterial O2 partial pressure (PaO2) and the PCr/Pi ratio or pHi; however, between PaO2 130-33 mmHg, metabolite changes were not significant. PCr/Pi and pHi decreased significantly when PaO2 was lowered below 33 and 28 mmHg, respectively. After recovery, metabolite ratios and pHi returned to base-line values within 5 min. During the early phases of hypoxia and recovery, there were large fluctuations in metabolites and pHi, indicating that mitochondrial reactions were not in a steady state. After several minutes of hypoxia or recovery, PCr/Pi and pHi stabilized, suggesting steady state kinetics for mitochondrial respiration. NMR is extremely sensitive to changes in mitochondrial oxygenation, and stable PCr/Pi and pHi indicate that O2 tension in cerebral mitochondria of the newborn lamb is constant between PaO2 of 30 and 140 mmHg.