Ageing is an aggravating factor leading to alterations in the biotransformation of drugs, and therefore their therapeutic efficacy and safety. In this review we discuss the influence of ageing on drug metabolizing enzymes in male Wistar rats. We report that drug metabolizing enzymes can be affected by ageing either by post-translational modifications or by transcriptional modifications. The post-translational modifications could be due to an increase of oxidative stress during ageing. Although it is now well established that transcriptional modifications are due to a change in the GH secretion profile in senescent rats, the intracellular mechanisms underlying these modifications are still unclear. In addition to the strong decrease in the activity of the main CYPs of male rats, we discuss the potential consequences on human drug metabolism in the elderly.