Glutathione metabolism was studied in rat liver during diethylnitrosamine (DEN) carcinogenesis. Some studies were also made in foetal rat liver. Endogenous GSH and non-protein thiols concentrations are increased in DEN-treated rats when compared to non-treated rats but no differences were found in cysteine, total thiols and protein thiols concentration. In foetal liver GSH concentration is only 35% of that in DEN-treated rat liver. The activities of several enzymes involved in glutathione metabolism are changed in DEN-treated rats. gamma-Glutamyl transferase activity and cysteine formation from GSH by liver homogenates is increased sevenfold. gamma-Glutamylcysteine synthetase activity, initial rate of [35S]cysteine incorporation in gamma-glutamylcysteine and initial rate of GSH formation from [35S]cysteine are increased two-fold. Cytosolic GSH S-transferase activity is increased twofold in DEN-treated rats and so GSH S-conjugates concentration is probably also increased. In foetal rat liver gamma-glutamyl transferase activity is about the same but gamma-glutamylcysteine synthetase activity is only 10% of that in DEN-treated rat liver. The increased GSH concentration in DEN-treated rat liver is probably due to the simultaneous increase in the activities of gamma-glutamyl transferase and gamma-glutamylcysteine synthetase. Blood plasma total glutathione is increased 1.4 times in DEN-treated rats, but no differences are found in GSH hepatic arteriovenous gradient. This associated with the increased gamma-glutamyl transferase activity suggests that sinusoidal GSH efflux is increased in DEN-treated rats.