The biliary route is very important for the elimination of some foreign compounds from the body. For many of these compounds, an increase in the rate at which they are excreted into the bile will decrease their toxicity and vice versa. A number of factors which are known to alter the biliary excretion of xenobiotics, as well as the current concepts of the physiological mechanisms responsible for the excretion of foreign compounds, have been enumerated. However, much remains still to be understood; essentially nothing is known at the subcellular level about the biliary excretion of foreign compounds. It has recently been concluded that our knowledge of the biliary excretion of compounds is about 40 years behind that of the renal excretion mechanism.