1. Changes in liver glutathione reductase and glutathione peroxidase activities in relation to age and sex of rats were measured. Oxidation of GSH was correlated with glutathione peroxidase activity. 2. Glutathione reductase activity in foetal rat liver was about 65% of the adult value. It increased to a value slightly higher than the adult one at about 2-3 days, decreased until about 16 days and then rose after weaning to a maximum at about 31 days, finally reaching adult values at about 45 days old. 3. Weaning rats on to an artificial rat-milk diet prevented the rise in glutathione reductase activity associated with weaning on to the usual diet high in carbohydrate. 4. In male rats glutathione peroxidase activity in the liver increased steadily up to adult values. There were no differences between male and female rats until sexual maturity, when, in females, the activity increased abruptly to an adult value that was about 80% higher than that in males. 5. The rate of GSH oxidation in rat liver homogenates increased steadily from 3 days until maturity, when the rate of oxidation was about 50% higher in female than in male liver. 6. In the liver a positive correlation between glutathione peroxidase activity and GSH oxidation was found. 7. It is suggested that the coupled oxidation-reduction through glutathione reductase and glutathione peroxidase is important for determining the redox state of glutathione and of NADP, and also for controlling the degradation of hydroperoxides. 8. Changes in glutathione reductase and glutathione peroxidase activities are discussed in relation to the redox state of glutathione and NADP and to their effects on the concentration of free CoA in rat liver and its possible action on ketogenesis and lipogenesis.