Changes in activity of some hepatic enzymes related to UDP-glucuronic acid conjugation (UDP-glucose dehydrogenase and UDP-glucuronyl transferase) and glutathione-related enzymes (glutathione reductase and glutathione peroxidase) were investigated in male and female Wistar rats as a function of age. UDP-glucose dehydrogenase activity showed a decrease in the ageing period in both sexes (26.4% and 37.7% in males and females respectively), and no sex differences were found in all the ages studied. The UDP-glucuronyl transferase (using p-nitrophenol as substrate) showed an age-dependent decrease in its activity for males, but an increase for female rats. A sex difference (male values were higher than female values) was observed only in young rats (1 and 3 months old). Glutathione peroxidase activity increased with age in both sexes (the activity found in male and female old rats was about 162% and 149% respectively to those found in adulthood), and a marked difference was observed between sexes in young and old rats (57.8% and 45.4% higher in females in young and old rats respectively). In contrast, the glutathione reductase activity showed a decrease in the ageing (39% in male and 35.5% in female) and the highest levels during lifetime was found in males.