Age-related changes in weight, morphology and physiology of the rodent liver influencing hepatic drug metabolism are reviewed. Next to the changes in liver weight/body weight ratio with age, the spontaneous occurrence of neoplastic and non-neoplastic lesions may be of particular importance. In addition, the decrease in liver blood blow with age diminishes the biotransformation capacity of the total liver. However, the albumin concentration in plasma and drug uptake do not play important roles, since they are unchanged or only slightly lower in old rats or mice. Drugs are generally metabolized by the liver in two phases: the so-called phase I and phase II metabolism. For most drugs, the phase I reaction is an oxidation. This reaction is catalyzed by cytochrome P-450, cytochrome b5 and NADPH-cytochrome c reductase. In microsomes, a decrease with age is generally observed in the cytochrome P-450 concentration and the NADPH-cytochrome c reductase activity, while there is no change in cytochrome b5. In addition, most microsomal drug-metabolizing enzymes decrease with age in male rats but not in females. The changes in enzyme activities in the male and female mouse are more complex. In fact, increases, decreases and no changes were found. Important phase II reactions are glutathione conjugation and glucuronidation; changes in both reactions with age seem to be of minor importance. Studies with hepatocytes isolated from male rats of different ages reveal that the monooxygenase system mediated metabolism of digitoxin and aflatoxin B1 decreases with age. It can be concluded that the observed decrease in the functional capacity of the monooxygenase system greatly determines the decrease in drug metabolism with age. However, it should always be kept in mind that, among others, the age-related changes in drug metabolism in rats are strongly sex dependent, which is not the case in man. Therefore, caution should be exercised in transferring these data to the human situation.