This review is focused on mechanisms of entry, detoxication and excretion of toxic compounds in mammals. The entry into blood stream and subsequent transport into target cells is discussed. Biotransformation in cells generally proceeds in two major phases, enzymatic and chemical modifications of toxic compounds. The major enzyme system involved in the biotransformation of xenobiotics is the group of cytochromes P-450. Formation of new functional groups, which is important for conjugation reactions, is catalyzed by the enzymes. The mechanisms of excretion of toxic compounds from the organism is discussed.