We investigated the metabolism of glutathione (GSH) and sulfur amino acids in various tissues of C57BL male mice fed on a 24.8% protein diet (normal protein diet, NPD), a 7.5% protein diet (low protein diet, LPD), and an amino acid supplemented diet (ASD), which contained 7.5% protein and the same levels of sulfur amino acids as NPD. Although the hepatic total GSH (TGSH) concentration was lower in LPD-fed mice than in NPD-fed mice, that in ASD-fed mice recovered to the level in NPD-fed mice. TGSH concentrations in the brain and plasma were highest in ASD-fed mice, whereas those in the kidney and blood were similar in the three dietary groups. Concentrations of L-methionine and L-cystathionine, an intermediate in the metabolism from L-methionine to L-cysteine, in the brain, kidney and plasma were highest in ASD-fed mice, whereas hepatic concentrations were similar. Brain uptake of ¹⁴C-L-methionine for 60 min was higher in LPD-fed mice than in NPD-fed mice, and was further enhanced in ASD-fed mice. Accordingly, this would account for the highest brain concentrations of sulfur amino acids in ASD-fed mice. Thus, the concentrations of TGSH and sulfur amino acids were markedly higher in various tissues in ASD-fed mice than in not only LPD- but also NPD-fed mice. These results suggest that the metabolism of GSH and sulfur amino acids would drastically change by a supplement of sulfur amino acids with a lowered protein diet, at least partly because of their excessive levels compared with the other amino acids.