Drug biotransformation and its therapeutic effect may be modified during ageing. Among different causative factors of ageing, the impairment of normal cellular functions by free radicals has been evoked as playing a critical role. The effect of age on the expression and activity of CYP2E1 and CYP3A was investigated in male Wistar rats of 3, 8, 11 and 18 months old. The total cytochrome P450 as well as the expression and the activity (midazolam oxidation) of CYP3A isoforms did not change until 18 months of age. Chlorzoxazone hydroxylation (CYP2E1 activity) increased from 3 to 8 months, remained constant between 8 and 11 months and then progressively decreased until 18 months. Interestingly, CYP2E1 microsomal protein followed the same enzyme activity profile from 3 to 8 months, but remained constant thereafter. The level of CYP2E1 mRNA did not change over the whole period. While the amount of proteins did not change after 8 months, their functionality may be affected by oxidative stress (increase in thiobarbituric acid reactive substances, decrease in reduced glutathione level). However, no changes in carbonyl protein content were observed. The decrease in CYP2E1 activity in rats after 11 months is most probably due to post-translational modifications of CYP2E1 proteins. Indeed, it may be correlated with an accumulation of oxidative damage. Since no change was observed in CYP3A activity or in their protein and mRNA content, it seems that such isoforms should be less affected by oxidative stress.