Gamma-glutamyl transpeptidase (gamma-GT) is the enzyme in the gamma-glutamyl cycle which mediates the first step of glutathione degradation. Recently, it has been reported that diethyl maleate, a glutathione-depleting agent, enhanced hepatic gamma-GT activity. In this study, we examined how gamma-GT mRNA was expressed after the depletion of glutathione by using the RT-PCR method and in situ hybridization technique with digoxigenin-labeled oligonucleotide probe. We could detect hepatic gamma-GT mRNA in the epithelial cell of bile duct and the cytoplasm of hepatocytes in the periportal area, even in control rats. Furthermore, we found that the expression of gamma-GT mRNA was expanded to hepatocytes of a whole lobule 12 hr after depletion of glutathione by diethyl maleate, followed by the return to the pretreatment condition under which gamma-GT mRNA appears to be controlled in correlation with hepatic glutathione levels. These results suggest that the depletion of glutathione may induce hepatic gamma-GT activity through an increased synthesis of its mRNA.