1. Precision-cut liver slices in dynamic organ culture, a novel in vitro technique, is described and applied to the study of hepatic drug metabolism in the rat. 2. These slices catalysed the oxidative O-deethylation of the substrate, 7-ethoxycoumarin, over 6 h incubation. In addition, the direct conjugation of 7-hydroxycoumarin with either sulphate or glucuronic acid was maintained over 6 h. 3. The formation of 7-hydroxycoumarin and the presence of the sulphate and glucuronide conjugates in slices exposed to 7-ethoxycoumarin demonstrated integrated phase I and phase II drug metabolizing activities in this system. 4. Minor modifications of the incubation system allowed for the metabolism of four volatile chlorinated benzenes: monochlorobenzene 1,2-, 1,3-, and 1,4-dichlorobenzenes to aqueous soluble metabolites. 5. The use of liver slices in dynamic organ culture as an alternative preparation for the study of xenobiotic metabolism is discussed.