An age-related decrease in the hepatic clearance of many drugs has been reported. Several mechanisms have been proposed, but only some are supported by hard evidence. Liver volume declines with age, as does hepatic blood flow--changes which may largely account for the reduced clearance of capacity- and flow-limited drugs, respectively. Age-related histological changes in the liver are minor and of uncertain significance; standard liver function tests do not change significantly with aging. There is, as yet, no direct evidence of a generalised fall in hepatic drug-metabolising enzyme activities in aging humans measured in vitro, but some in vivo studies suggest that certain very specific cytochrome P450 isoenzymes may be affected by aging, especially in men. Finally, there may be an age-related decline in the response to environmental influences.