Since glutathione (GSH) protects against oxidative stress, we determined the regulation of cellular GSH by ionizing radiation in human hepatoblastoma cells, HepG2. The levels of GSH increased in irradiated HepG2 due to a greater gamma-glutamylcysteine synthetase (gamma-GCS) activity, which was paralleled by gamma-GCS heavy subunit chain (gamma-GCS-HS) mRNA levels. Transcription of deletion constructs of the gamma-GCS-HS promoter cloned in a reporter vector was associated with activator protein-1 (AP-1), consistent with the DNA binding of AP-1 in nuclear extracts of irradiated HepG2. Hence, the transcriptional regulation of gamma-GCS by ionizing radiation emerges as an adaptive mechanism, which may be of significance to control the consequences of the oxidative stress induced by radiation.