The mechanism of the elevation of intracellular glutathione induced by low-dose γ-rays was examined in RAW 264.7 cells. The expression of mRNA for γ-glutamylcysteine synthetase (γ-GCS) increased soon after γ-ray (0.5 Gy) irradiation, and peaked between 3 h and 6 h post-irradiation. A dose of 0.25 to 0.5 Gy was optimum for induction of γ-GCS mRNA expression at 3 h post-irradiation. The effect of inhibitors of activator protein-1 (AP-1) and nuclear factor κB (NF-κB) on the radiation-induced γ-GCS gene expression was then examined. The induction of γ-GCS mRNA expression was significantly suppressed when AP-1 DNA binding, but not NF-κB DNA binding, was inhibited. Finally, electrophoretic mobility shift assay showed that the low-dose radiation markedly increased the DNA binding of AP-1, but not NF-κB, soon after irradiation. These results suggest that the increase of glutathione levels in RAW 264.7 cells by low-dose γ-ray irradiation is mediated by transcriptional regulation of the γ-GCS gene, predominantly through the AP-1 binding site in its promoter.