The clearance of many drugs by the liver is reduced in the elderly, and this accounts, at least in part, for the greater frequency of adverse drug reactions in this population. Ageing in experimental animals is accompanied by a decline in the activities of certain microsomal monooxygenase enzymes; this has been said to be the reason for decreased drug elimination. However, recent data suggests that this is probably not the case in humans. Reduced size and blood flow are important features of the ageing liver, and probably account for most of the age-related changes in drug metabolism. Major decrements in hepatic function do occur in frail elderly individuals, although the mechanisms underlying this are at present unclear. Future studies of drug metabolism and ageing must carefully define patient groups to allow full interpretation of data.