1. There appears to be a statistically highly significant difference in the blood glutathione (GSH) level between groups of mental patients and adequate individuals.2. This difference is greatest in catatonic dementia præcox. It is also marked in hebephrenic and paranoid præcox, and in general paresis. It is less marked but still statistically significant in manic-depressive psychosis. In the small number of cases of epilepsy examined it was not statistically significant.3. Diurnal variation and exercise have been ruled out as influences affecting the validity of conclusions based on the data accumulated in this study.