The changes of the activities of serum CK,LDH,HBKH and AST of 11 soldiers having stayed in simulated 3500m and 4500m were observed.It was found that the activities were significantly increased in the order of CK,LDH and HBDH because of hypoxia but the changes of AST was of no statistical significance.CK/LDH ratio and pulse rate showed similar charges to serum enzymes but the changes of arterial oxygen saturation and working efficiency were the opposite to those of serum enzymes.These findings suggest that serum CK and LDH are the indicators of choice to evaluate human reactions to high altitude.