The levels of reduced glutathione in severe glucose-6-phosphate dehydrogenase deficient red blood cells, were found, when assayed immediately after blood withdrawing, almost equal to those of normal human erythrocytes. On the contrary, if the blood samples were stored at 4 degrees C for 4 days (or more) either as whole blood or as washed erythrocytes with or without glucose, the glutathione concentration of the enzyme-deficient cells decreased to half the initial value. After a mild t-butylhydroperoxide (t-BHP) treatment, only glucose-6-phosphate dehydrogenase deficient red cells exhibited a drastic decrease of glutathione, the normal ones being almost unaffected. If t-BHP-treated erythrocytes were incubated in the presence of glucose or of oxidized glutathione, a full recovery of the inital glutathione concentration was detected only in normal samples. Glucose could not be replaced by any other sugar. When the oxidative stress was induced by addition of methylene blue (MB), the behaviour was similar but less marked, it was however impossible, in this case, to restore the normal glutathione levels, through a subsequent incubation of the MB-treated erythrocytes with oxidized glutathione. This discrepancy can be explained by the finding that a marked inhibition of glutathione reductase was observed in MB-treated erythrocytes, while t-BHP exposure had no direct effect on this enzyme.