A method of measuring of glutathione peroxidase activity using H 2 O 2 was adapted for homogenates of murine brains. If the amount of reduced glutathione was at the constant level of 0.55 mM, the concentration of H 2 O 2 of 0.192 mM was saturating for glutathione peroxidase of murine brain and was selected as an optimal concentration for the estimation of enzyme activity in tris-HCl buffer with addition of NaN 3 and EDTA (pH 8.5) at the incubation temperature of 37 o C. The homogenates were diluted with the reaction mixture by 10.4 times. The duration of incubation did not exceed 60 sec, if 13% homogenate was used. The experiment, in which this method has been applied, showed increased activity of glutathione peroxidase in the brain of mice treated with a derivative of acetaldehyde ammonia during long-term intermittent normobaric hypoxia. These data might reflect activation of glutathione peroxidase.