The cytochrome P450 3A (CYP3A) is a member of the cytochrome P450 monooxygenase superfamily. The multidrug resistance 1 (MDR1) gene belongs to the ATP-binding cassette (ABC) family. Pregnane X receptor (PXR) is a nuclear receptor that regulates its target gene transcription in a ligand-dependent manner. Lipopolysaccharide (LPS)-induced downregulation of PXR, CYP3A and MDR1 in liver has been demonstrated in a series of studies. However, it is not clear whether LPS represses the expression of PXR, CYP3A and MDR1 in placenta. In the present study, we investigated the effects of LPS on the expression of PXR, cyp3a11 and mdr1a in mouse placenta. Pregnant ICR mice were injected intraperitoneally with different doses of LPS (0.1-0.5 mg/kg) on gestational day (gd) 17. Placental PXR, cyp3a11 and mdr1a mRNA levels were determined at 12 h after LPS treatment using RT-PCR. Results showed that LPS significantly downregulated PXR, cyp3a11 and mdr1a mRNA levels in a dose-dependent manner. LPS-induced downregulation of PXR, cyp3a11 and mdr1a mRNA in placenta was significantly attenuated after pregnant mice were pre- and post-treated with alpha-phenyl-N-t-butylnitrone (PBN), a free radical spin trapping agent. Additional experiments revealed that LPS increased lipid peroxidation and proinflammatory cytokine expressions in mouse placenta, all of which were also attenuated by PBN. Furthermore, LPS-induced downregulation of PXR, cyp3a11 and mdr1a mRNA in mouse placenta was prevented by N-acetylcysteine (NAC). NAC also inhibited LPS-initiated lipid peroxidation, GSH depletion and proinflammatory cytokine expressions in mouse placenta. These results indicated that LPS downregulates placental PXR, cyp3a11 and mdr1a mRNA expressions. Reactive oxygen species (ROS) may be involved in LPS-induced downregulation of PXR, cyp3a11 and mdr1a in mouse placenta.