Abstract:The effect of mild hypoxic hypoxia on brain metabolism and acetylcholine synthesis was studied in awake, restrained rats. Since many studies of hypoxia are done with animals anesthetized with nitrous oxide (N2O), the effects of N2O were evaluated. N2O (70%) increased the cerebral cortical blood flow by 33% and the cortical metabolic rate of oxygen by 26%. In addition, the synthesis of acetylcholine in N2O‐anesthetized animals, measured with [U‐14C]glucose and [1‐2H2,2‐2H2]choline, decreased by 45 and 53%, respectively. Consequently, mild hypoxia was studied in unanesthetized rats. Control rats breathing 30% O2(partial pressure of oxygen, Pao2= 120 mm Hg) were compared with rats exposed to 15% O2(Pao2= 57 mm Hg) or 10% O2(Pao2= 42 mm Hg). The synthesis of acetylcholine, measured with [U‐14C]glucose, was decreased by 35 and 54% with 15% O2and 10% O2respectively; acetylcholine synthesis, measured with [1‐2H2,2‐2H2]choline, was decreased by 50 and 68% with 15% O2and 10% O2respectively. Animals breathing either 15% or 10% O2had normal cerebral metabolic rates of oxygen but had increased brain lactates and increased cortical blood flows compared with animals breathing 30% O2. These results show that even mild hypoxic hypoxia impairs acetylcholine synthesis, which in turn may account for the early symptoms of brain dysfunction associated