Biliary excretion is an important route for the elimination of some drugs and drug metabolites in man. The factors which determine elimination via the biliary tract include characteristics of the drug such as chemical structure, polarity and molecular size as well as characteristics of the liver such as specific active transport sites within the liver cell membranes. A drug excreted in bile may be reabsorbed from the gastrointestinal tract or a drug conjugate may be hydrolysed by gut bacteria, liberating original drug which can be returned to the general circulation. Enterohepatic circulation may prolong the pharmacological effect of certain drugs and drug metabolites, but the quantitative importance of this in man appears to be less than in animals. Biliary elimination may play a role in the interindividual differences in drug response observed in healthy subjects and in patients with certain diseases. Cholestatic disease states, in which normal bile flow is reduced, will influence drug elimination by this route resulting in increased risk of drug toxicity. Bile may serve as an alternate route of elimination in renal failure, but this has not been determined in man. Lack of reliable information regarding the biliary excretion of drugs in man is partly due to the relative inaccessibility of the human biliary tract. Most studies of drug excretion in human bile have been performed in post-surgical patients with T-tube drainage. This method of bile collection is not ideal because bile flow and composition are often severely altered during the period of study, not all bile is collected and enterohepatic circulation is partially interrupted. Recent advances in the methods of collection of bile may improve future studies of drug excretion in human bile.