Total glutathione (GSH and GSSG) level, and the activities of gamma-glutamylcysteine synthetase, gamma-glutamyltranspeptidase (gamma-GTP), glutathione S-transferase (GST), glutathione peroxidase (GSH-Px) and glutathione reductase (GR) in the liver were investigated in rats, mice, guinea pigs and hamsters. Hepatic GSH level in rats, mice, guinea pigs and hamsters were 7.1, 7.8, 3.5 and 5.4 mM, respectively. The lower level of GSH in guinea pigs seems to be in part attributed to the higher activity of hepatic gamma-GTP, an enzyme which catalyzes GSH breakdown. Moreover, a marked species difference in the activities of GST, GSH-Px and GR was also observed. A 48 h-fasting resulted in a decrease of GSH and GSSG levels in rats, mice and guinea pigs, but not in hamsters. In addition, both nicotineamide adenine dinucleotide phosphate- and ascorbate-dependent lipid peroxidation produced by 9000 X g supernatant fraction in fasted animal species occurred most highly in the rat followed by hamster and guinea pig, and almost undetectable in mice. Thus, it suggests that the occurrence of lipid peroxidation in fasted animals may not be related to the hepatic GSH level, and rather, a lack of occurrence of lipid peroxidation in fasted mice may be due to the increased activity of GSH-Px activity.