Hepatic cytochromes P-450 (CYP) are well characterized drug and xenobiotic metabolizing enzymes that are extensively regulated by genetic and environmental factors. Inflammatory mediators, including interleukins (ILs), interferons (IFNs), and tumor necrosis factor-alpha (TNF-alpha), have been shown to down-regulate several CYP isoforms; however, elucidation of the inflammatory mediators that are responsible for specific CYP down-regulation is difficult. The purpose of this experiment was to evaluate the role endogenous TNF-alpha plays in the regulation of liver CYP expression after endotoxin administration. Mice deficient in the p55 and p75 TNF receptors and wild-type mice were given Gram-negative bacterial lipopolysaccharide (LPS) and killed 24 h after administration. CYP analysis indicates that LPS decreases CYP1A, CYP2B, CYP3A, and CYP4A independently of TNF-alpha. CYP2D9 and CYP2E1 activities show differential responses to LPS between wild-type and TNF p55/p75 receptor knockout mice, indicating the down-regulation of CYP2D9 and CYP2E1 is differentially modulated by TNF-alpha expression. Furthermore, TNF-alpha appears to affect the constitutive expression of CYP2D9 and CYP2E1. To date, this is the first evidence suggesting that a proinflammatory cytokine is involved in the constitutive regulation of drug-metabolizing enzymes.