Glucuronidation is a crucial pathway of metabolism and excretion of endogenous compounds and xenobiotics. UDP-glucuronyltransferases, UGT, catalyse transformations of bilirubine, steroids and thyroid hormones, bile acids as well as exogenous compounds, including drugs, carcinogens, environmental pollutants and nutrient components. From therapeutic point of view, the participation of UGTs in drug metabolism is of particular significance. Polymorphism of UGT1A and UGT2B genes resulted in various susceptibility of substrates to conjugation with glucuronic acid. Deactivation of xenobiotics and the following excretion of hydrophilic conjugates is a common task of glucuronidation, which should lead to detoxification. However, a lot of glucuronides were known, which expressed the comparable or even higher reactivity than that of the native compound. There are, among others, acyl glucuronides of carboxylic acids, morphine 6-O-glucuronide or retinoid glucuronides. They are able to bind cellular macromolecules with low or high strength and, as a consequence, their toxicity is saved or even increased, respectively.