Purpose and Hypothesis: During inflammation, the expression and activity of key phase I and phase II drug metabolizing enzymes (DMEs) and transporters are altered in the liver, leading to impaired drug metabolism and clearance. Inflammation in the liver is primarily mediated by the Toll-like receptors (TLRs). It is known that the gene expression of DMEs and transporters is regulated by basal transcription factors and the nuclear receptors, constitutive androstane receptor (CAR) and pregnane X receptor (PXR). We have shown that activation of TLR2 by lipotechoic acid (LTA), which is a gram positive bacterial component, leads to a rapid and profound suppression of CAR and its target DME and transporter genes. We hypothesize that down-regulation of hepatic DMEs and transporters is mediated by CAR. Methods: Adult, male (6-8 weeks old) CAR+/+ and CAR-/- mice (C57BL/6) were treated with saline or LTA (6 mg/kg) for 4 or 8 hours. Liver tissues were harvested and RNA expression was analyzed by real-time PCR analysis. For in vitro studies, primary hepatocytes were isolated from CAR+/+ and CAR-/- mice and treated with saline or LTA (50ng/ml) for 8 hours. RNA was analyzed by real-time PCR analysis. Results: As expected, down-regulation of gene expression of the DMEs, Cytochrome P450 (Cyp) 2b10, Cyp2a4 and Amine-N-Sulfotransferase was significant (~50-60%) in LTA-treated CAR+/+ mice. Similarly, the gene expression of the drug transporter, multidrug resistance-associated protein (MRP) 2 was significantly down-regulated (30-40%) by LTA treatment in CAR+/+ mice. The down-regulation of these DMEs and transporters was attenuated in the CAR-/- mice treated with LTA. LTA-mediated reduction in Cyp3a11 RNA and catalytic activity was attenuated in the CAR-/- mice. The basal levels of Cyp2b10 were significantly reduced in the CAR-/- mice as compared to the CAR+/+ mice. A similar down-regulation (~40-45%) was observed in primary hepatocytes isolated from CAR+/+ mice treated with LTA whereas this down-regulation was attenuated in hepatocytes from CAR-/- mice. Conclusion: For the first time, we show that the nuclear receptor CAR is involved in mediating the down-regulation of gene expression of select DMEs and transporters during inflammation induced by gram positive bacterial components.