Simulation of human plasma concentrations of the teratogen thalidomide and its human metabolites is possible with a simplified physiologically-based pharmacokinetic model based on data obtained in mice with humanized liver. Pre-treatment with orally administered thalidomide enhanced the model substrate midazolam clearance in humanized-liver mice due to human cytochrome P450 3A4 induction, as also evident in in vitro studies in human hepatocytes. In vivo non-specific hepatic protein binding parameters of metabolically activated [14C]-5-hydroxythalidomide and subsequent metabolic activation in humanized liver mice can be analyzed by accelerator mass spectrometry. In this study, more relevant human placenta preparations and placental BeWo cells showed low but detectable P450 3A4/5 mRNA expression and drug oxidation activities. Human placental microsomal fractions from three subjects showed detectable midazolam 1´- and 4-hydroxylation and thalidomide 5-hydroxylation activities. Thalidomide significantly induced P450 3A4/3A5 and pregnane X receptor (PXR) mRNA levels 2- to 3-fold in human placental BeWo cells cultured with a modified medium containing 5% charcoal-stripped fetal bovine serum. Under these modified conditions, thalidomide also significantly induced midazolam 1´-hydroxylation and thalidomide 5-hydroxylaion activities 3-fold. Taken together, activation of thalidomide to 5-hydroxythalidomide with autoinduction of P450 3A enzymes in human placentas, as well as livers, is suggested in vivo.