Glutathione (GSH) is a powerful and general antioxidant found in cell of various animals. Since its concentration in central nervous system has been thought to related to psychiatric disease, several studies has carried out but the results are still controversial. Here, we investigated GSH diurnal fluctuation of healthy subjects and compared with the results of the reproducibility study. Our results shows GSH concentration in brain fluctuates by 10 to 20% during the daytime. This finding indicates that conditions of MRS examination must be regulated strictly and the fluctuation should be took into account at interpretation of GSH studies.