In the result of liver detoxification, xenobiotics change into more water soluble and thus easier for excretion from the body. It is convenient to consider this process as occurring in two phases. In phase I, the major reactions involved are hydroxylation, catalyzed by monoxygenases. In phase II, the preliminary modified xenobiotics after conjugation with some specific metabolites are transformed into less toxic and more soluble end-products. Recently, antiporter activity of MDR1 (MultiDrug Resistence) gene products in enterocytes was recognized as important stage in detoxification of xenobiotics, and definied as phase III of this process.