In humans, CYP3A represents one of the most important subfamilies of the CYP superfamily and is the most abundantly expressed CYP in human liver. It is involved in the biotransformation of ~50% of metabolized drugs, and as a result, drug-drug interactions (DDI) associated with modulation of CYP3A-mediated metabolism are of clinical importance. The PXB-Mouse®, a chimeric mouse with a humanized liver, can be used as a predictor of human hepatic elimination and of the potential for DDI. Midazolam (MDZ) and ketoconazole (KCZ) are widely used as a probe and inhibitor, respectively, for evaluating the metabolism of CYP3A4 clinically and in vitro. The aim of this study was to evaluate the in vivo pharmacokinetics (PK) of MDZ, administered as a single p.o. dose, and its primary metabolite, 1’OH-MDZ, alone and when co administered with KCZ in the PXB-Mouse® and SCID mouse as a control.