We examined the mechanisms of the elevation of glutathione level induced in macrophage-like RAW 264.7 cells by low closes of gamma-rays. The level increased soon after exposure of the cells to 50 cGy of gamma-rays, peaked between 3 hours and 6 hours and rearmed almost to the time 0 value by 24 hours post-irradiation. Doses between 25 and 100 cGy significantly increased the glutathione level at 4 hours post-irradiation. However there was no significant elevation at doses of more than 100 cGy or less than 25 cGy. When the effect of dose I-ate was examined at a constant absorbed dose of 50 cGy, dose rates Of more than 50 cGy/minute significantly increased the GSH level at 4 hours post-irradiation. It was also shown that the elevation of glutathione level in cells irradiated with low doses of gamma-rays followed the induction of mRNA coding for gamma-glutamylcysteine synthetase (gamma-GCS) a rate-limiting enzyme of the de novo glutathione synthesis pathway When the cells were exposed to the radiation in the presence of genistein, calphostin C or nifedipine, the elevations of glutathione and gamma-GCS mRNA expression were both mostly blocked. EGTA also strongly inhibited these elevations. These results suggest that the tyrosine kinase, calcium channel and protein kinase C activities play an essential role in the law-dose-radiation-induced elevation of cellular glutathione.