Among the in vitro models developed to investigate drug metabolism isolated hepatocytes have become the most powerful model. Human hepatocytes can be prepared from whole livers and surgical wedge biopsies. When placed in culture they retain their specific drug metabolizing activities including inducible cytochrome P450 enzymes for several days. Primary human hepatocyte cultures are now increasingly used for studying drug behavior during preclinical development, e.g. drug interactions, stereoselective drug metabolism and drug metabolic profiles. As a rule there is a good in vivo/in vitro correlation in drug biotransformation activity. The main metabolites found in vivo are recovered in vitro. However, quantitative differences are frequently observed.