Prior animal studies have indicated that levels of oxidized glutathione (GSSG) in brain and other organs are low, comprising several percent, or less, of the total glutathione. An exception is seen in reports for autopsy and biopsy specimens of human brain, where very high levels of GSSG have been indicated. The latter observations imply an unusual redox state for human brain. In the current study, GSSG and reduced glutathione (GSH) were measured in monkey brain and autopsy specimens of human brain. Levels of GSSG were 1.2% or less, of the total glutathione. These results are similar to those for rodent brain, but disagree with earlier reports concerning human brain.