When a pollutant enters a cell it may trigger certain biochemical responses which have evolved either to break the chemical down (Chapter 7) or to store it in such a way that it is hidden' within a compartment, preventing in terference with essential biochemical reactions within the cell. For example, in invertebrates there are clearly defined pathways for metal detoxication (Hopkin, 1989, 1990; Dallinger, 1993). Perhaps the simplest case is the epithe lium of the digestive system of terrestrial in- vertebrates which is usually only one cell in thickness and acts as a barrier between the in ternal environment of the animal (i.e. the blood bathing the organs) and the food in the lumen Therefore, storage mechanisms and/or meth ods of exclusion have to be extremely efficient because terrestrial invertebrates (unlike aquatic organisms) are not able to excrete xenobiotics from the blood into the external medium across the respiratory surfaces if they are taken up to excess. In land mammals, lipophilic organics are converted to water-soluble metabolites and conjugates which are then excreted in the bile and urine.