Recent research indicates that physical exercise and fitness are new host factors with impact on hepatic drug metabolism, contributing to the intra- and interindividual variation in drug response.Moderate to heavy physical exercise for a few hours reduces liver blood flow as assessed by indocyanine green clearance, leading to a decreased elimination of drugs exhibiting flow-limited metabolism (high clearance drugs) such as lignocaine (lidocaine). However, hepatic elimination of drugs exhibiting capacity-limited metabolism (low clearance drugs) such as antipyrine (phenazone), diazepam and amylobarbitone (amobarbital) is not affected by acute physical exercise.Improved physical fitness as expressed by the maximum oxygen uptake seems to increase the elimination rate of the low clearance drug antipyrine and possibly also aminopyrine, while investigations of the biotransformation of high clearance drugs are contradictory.The sum of research in this recent field is rather limited and the mechanism whereby changes in physical fitness influence hepatic drug metabolism needs to be established. It is not known if other liver functions are changed. If the findings also apply for drugs with a low therapeutic index, there may be a risk of exercise-induced changes in drug efficacy and toxicity.It is suggested that future studies on host factors influencing drug metabolism should include information on physical activity.