B6C3F1 mice were treated per os with either normal saline or N-nitrosodiethylamine (NDEA) (0.01, 0.1, 1.0 or 5.0 mg/kg body weight) daily for 21 days. On day 22nd of the experiment, the animals were whole-body g-irradiated (10 Gy) and examined at 3.5 days post-radiation exposure. Pretreatment of mice with NDEA at the lowest dosage (0.01 and 0.1 mg/kg) increased thiobarbituric acid-reactive substances (TBARS) and catalase (CAT) activity in the liver. Since the agent at the highest doses (1.0 and 5.0 mg/kg) did not have any effect(s) on TBARS, it was associated with the selective increase of thiol (SH) groups and GSH-linked anti-oxidant enzyme activities such as glutathione peroxidase (GPX), transferase (GST) and reductase (GR). g-Irradiation decreased TBARS and increased superoxide dismutase (SOD) and GPX activity in NDEA-treated mice. Simultaneously, g-rays did not have any effect(s) on GST and GR enzymes, and it slightly decreased SH groups and CAT activity. Results of the present study indicate that NDEA can promotes lipid peroxidation in mice liver. g-Irradiation of mice at a dose of 10 Gy is enable to modify the activity of hepatic anti-oxidant enzymes, which in turn can lead to the reduction of NDEA-induced lipid peroxidation and/or pro-oxidant shift(s). The anti-oxidant enzymes such as SOD and GPX are suggested to be mainly involved in this process(s).