The effect of acute hypoxia on the changes of plasma cortisol level and hippocampus NE level of cat during inhalation of 21%, 10.4% or 7.9% O2 were investigated. It was found that both 10.4% and 7.9% O2 inhalation caused a gradual increase in plasma cortisol level reaching a plateau in 30-45 min time. While the contents of NE in VHIP and DHIP showed significant decrease 30 min after hypoxia. The degree of both these changes are not related to the hypoxic level of the inhaled O2. The results show that acute hypoxic stress would induce some increase in plasma cortisol level and some decrease in hippocampus NE content.