The role of P450s in xenobiotic metabolism, toxicity, and carcinogenicity has been studied for many years by using in vitro approaches and limited in vivo investigations. Genetic analysis to study the effects of xenobiotics in intact animals has only recently been carried out by use of gene knockout mice. Mice lacking expression of these enzymes have no or only modest phenotypes, indicating that their xenobiotic-metabolizing enzymes are not critical for mammalian development or physiological homeostasis. The null mice have been used to study the roll of xenobiotic-metabolizing enzymes in chemical toxicity and carcinogenicity. There are marked species differences in the expression and catalytic activities of P450s that metabolize xenobiotics, and this complicates the extrapolation of data obtained in rodents for use in drug development and human risk assessment. This is especially notable between mice and rats, commonly used experimental models, and humans. To begin to develop more predictive models, P450 humanized mice were produced and characterized by using genomic clones containing the complete coding and regulatory regions of genes, as transgenes. Humanized lines expressing CYP2D6 and CYP3A4 human P450 were characterized and found to accurately express human P450 proteins and catalytic activities at levels comparable to or higher than the corresponding activities found in human tissues. These novel mouse lines offer the opportunity to predict human drug and carcinogen metabolism and disposition and to search for endogenous substrates for human P450s.