Multidrug resistance continues to be a major impediment to successful chemotherapy in cancer patients. One cause of multidrug resistance is enhanced expression of the mdr1 gene, but the precise factors and physiological conditions controlling mdr1 expression are not entirely known. To gain a better understanding of mdr1 transcriptional regulation, we created a unique mouse model that allows noninvasive bioimaging of mdr1 gene expression in vivo and in real time. The model uses a firefly luciferase (fLUC) gene inserted by homologous recombination into the murine mdr1a genetic locus. The inserted fLUC gene is preceded by a neo expression cassette flanked by loxP sites, so that Cre-mediated recombination is required to configure the fLUC gene directly under the control of the endogenous mdr1a promoter. We now demonstrate that the mdr1a.fLUC knock-in is a faithful reporter for mdr1a expression in naive animals, in which fLUC mRNA levels and luminescence intensities accurately parallel endogenous mdr1a mRNA expression. We also demonstrate xenobiotic-inducible regulation of mdr1a.fLUC expression in real time, in parallel with endogenous mdr1a expression, resulting in a more detailed understanding of the kinetics of mdr1a gene induction. This mouse model demonstrates the feasibility of using bioimaging coupled with Cre/loxP conditional knock-in to monitor regulated gene expression in vivo. It represents a unique tool with which to study the magnitude and kinetics of mdr1a induction under a variety of physiologic, pharmacologic, genetic, and environmental conditions.