A vast number and variety of xenobiotics appear in the bile. For some this is a final excretory process, for others it is merely one step in the active enterohepatic circulation. For still others it may be a vital step in a toxicologic or carcinogenic process. During the past ten years there has been a steady accumulation of observations in the literature bearing on biliary excretion mechanisms. Phenomena such as molecular weight thresholds and other aspects of species variation as well as response to inducing agents are described in many papers and much speculation is available as to their meaning. The clinical significance of this work is still somewhat dependent upon results obtained from lower animals, although studies occasionally appear on patients who have temporary bile drainage subsequent to surgery. It is important that efforts persist in obtaining data in humans since extrapolation from lower animals in the area of drug disposition is often precarious. The basic physiological and biochemical mechanism governing the biliary fate of drugs and other xenobiotics have yet to be elucidated fully. Perhaps the use of drugs and other pharmacological tools will hasten progress toward this goal.