Introduction Elimination from the body is very important in determining the potential toxicity of a xenobiotic.  When a toxic xenobiotic (or its metabolites) is rapidly eliminated from the body, it is less likely that they will be able to concentrate in and damage critical cells.  The terms excretion and elimination are frequently used to describe the same process whereby a substance leaves the body.  Elimination, however, is sometimes used in a broader sense and includes the removal of the absorbed xenobiotic by metabolism as well as excretion.  Excretion, as used here, pertains to the elimination or ejection of the xenobiotic and it's metabolites by specific excretory organs. Except for the lung, polar (hydrophilic) substances have a definite advantage over lipid-soluble toxicants as regards elimination from the body.  Chemicals must again pass through membranes in order to leave the body, and the same chemical and physical properties that governed passage across other membranes applies to excretory organs as well. Toxicants or their metabolites can be eliminated from the body by several routes.  The main routes of excretion are via urine, feces, and exhaled air.  Thus, the primary organ systems involved in excretion are the urinary system, gastrointestinal system and respiratory system.  A few other avenues for elimination exist but they are relatively unimportant, except in exceptional circumstances.