Reduced glutathione, enzymes involved in its metabolism and other cytosolic activities were evaluated in liver preparations of Wistar rats fed with a diet supplemented with 2-acetylaminofluorene (0.05%) and/or with glutathione or N-acetyl-L-cysteine (0.1%). The treatment lasted 4 cycles, each composed of 3 weeks of special diet followed by 1 week of standard diet. The carcinogen produced a considerable increase in gamma-glutamyl transpeptidase in liver homogenates at cycles III and IV, with an irreversible trend which was not discontinued even during the weeks of standard diet. Moreover, generally from cycle I, 2-acetylaminofluorene stimulated several enzyme activities in the liver cytosol, such as glutathione S-transferase, glutathione reductase, glucose 6-phosphate dehydrogenase, NADH- and NADPH-dependent diaphorases. Administration of the two aminothiols to untreated rats resulted in a significant enhancement of glutathione peroxidase, glucose 6-phosphate dehydrogenase and diaphorases. In 2-acetylaminofluorene-treated rats, both thiols further stimulated glutathione S-transferase during the last treatment cycles and attenuated gamma-glutamyl transpeptidase activity, which however was not sufficient to thoroughly counteract the liver lesions due to the massive feeding of the carcinogen. Hepatocellular glutathione was enhanced during the last cycle of treatment with 2-acetylaminofluorene, and was further increased by co-administration of exogenous glutathione.