Hepatic endogenous hydrogen peroxide (H2O2) in bile of selenium-deficient rats (SeD) was for the first time found using the electron spin resonance (ESR) spin-trap technique, and the relationship between glutathione peroxidase (GPX) activity and H2O2 amount is discussed. Normal rats and four groups of rats fed a selenium-deficient diet with different feeding periods were examined. The results showed that the GPX activity decreased depending on the feeding period with the selenium-deficient diet and that the hepatic endogenous H2O2 amount in the bile of the rats fed the selenium-deficient diet for the longest period (a week before birth to 8 weeks old) was drastically higher than those in other groups of rats (P < 0.005). We found that generation of H2O2 due to the decrease in the GPX activity has a threshold value. The results suggest that an exposure to selenium deficiency for long term will cause oxidative stress.