Signal transducer and activator of transcription (Stat), a family of transcriptional factors, has been demonstrated to play a critical role in gene regulation in response to inflammatory cytokines, such as interferon and interleukin-6. Inflammatory cytokines and bacterial endotoxin are known to suppress, in most of cases, the constitutive or induced cytochromes P450 (P450) in animals and humans. However, it is not clear if the suppression of P450 by cytokines is through the Stat-signaling pathway. In the present study, we determined whether Stat1 is involved in lipopolysaccharide (LPS)-mediated modulation of P450 in mouse liver. In both Stat1(+/+) (wild type) and Stat1(-/-) (null) mice, a single dose of LPS treatment (1 mg/kg of body weight, i.p.) significantly reduced the expression of CYP3A11, 2C29, and 1A2 mRNA to 8 to 40% of the control levels as determined by real-time quantitative reverse transcription-polymerase chain reaction. The reduction was supported by Western blot analysis. In contrast, LPS significantly induced the level of CYP4A10 mRNA in both Stat1(+/+) (338% of control) and Stat1(-/-) mice (264% of control). Although suppression of mRNA levels of CYP2E1, and 2D9 was not observed in either LPS-treated Stat1 null or wild-type animals, LPS treatment resulted in a reduction of CYP2E1 protein content, which was more significant in Stat1(+/+) (23% of control) than in Stat1(-/-) mice (67% of control). Consistent with this result, the chlorzoxazone 6-hydroxylase and lauric acid 11-hydroxylase activities, as CYP2E1 representative activities, were reduced markedly by LPS in Stat1(+/+) but not in Stat1(-/-) mice. The ethoxyresorufin O-deethylase activity, as a representative CYP1A activity, was also reduced significantly only in LPS-treated Stat1(+/+) mice. These data clearly demonstrate that LPS-mediated modulation of CYP3A11, 2B10, 2C29, 1A2, and 4A10 in mouse liver is Stat1-independent. However, the significant difference between the LPS-treated Stat1(+/+) and Stat1(-/-) mice in the levels of CYP2E1 protein and activity as well as in the activity level of CYP1A suggests that Stat1 may be indirectly involved in the post-transcriptional modulation of these two mouse P450 enzymes.