Xenobiotics, including drugs, can influence cytochrome P450 (CYP) activity by upregulating the transcription of CYP genes. To minimize potential drug interactions, it is important to ascertain whether a compound will be an inducer of CYP enzymes early in the development of new therapeutic agents. In vivo and in vitro studies are reported that demonstrate the use of liver and intestinal slices as an in vitro model to predict potential CYP induction in vivo. Rat liver slices and intestinal slices were incubated, for 24 h and 6 h, respectively, with beta-naphthoflavone (betaNF), phenobarbital (PB) or dexamethasone (DEX). In an in vivo study, rats were treated with the same compounds for 3 days. In vivo and in vitro CYP mRNA levels were measured by using real-time quantitative reverse transcription-polymerase chain reaction (RT-PCR). In addition, CYP enzyme activities were determined in rat liver slices after 48 h incubation. In both rat liver and intestinal slices, betaNF significantly induced CYP1A1, CYP1A2 and CYP2B1 mRNA levels. PB significantly induced CYP2B1. In liver slices a minor induction of CYP1A1 and CYP3A1 by PB was observed, whereas DEX significantly induced CYP3A1, CYP2B1 and CYP1A2 mRNA levels. The induction profiles (qualitative and quantitative) observed in vivo and in vitro are quite similar. All together, these data demonstrate that liver and intestinal slices are a useful and predictive tool to study CYP induction.