Objective: Although hepatic cytochrome P-450 protein concentrations are altered following endotoxin shock, changes in P-450 isoforms in sepsis have not been fully investigated. The aim of this study was to determine whether the major P-450 isoform in rat liver (i.e., CYP1A2) is down-regulated during the progression of sepsis and, if so, whether reduction of P-450 enzyme system plays an important role in the inflammatory response. Design: Prospective, controlled, and randomized animal study. Setting: A university/institute research laboratory. Subjects: Male adult Sprague-Dawley rats were subjected either to polymicrobial sepsis by cecal ligation and puncture (CLP) or to sham operation followed by the administration of normal saline solution (i.e., fluid resuscitation). Interventions: P-450 isoforms in the liver (i.e., CYP1A2 and 4A1) were determined using reverse transcription polymerase chain reaction and Western blot analysis at various time points after CLP. Measurements and Main Results: The results indicate that CYP1A2 messenger RNA expression decreased significantly at 10 and 20 hrs whereas its protein concentrations decreased at 20 hrs after the induction of sepsis. In contrast, CYP4A1 messenger RNA and protein concentrations were not altered even at 20 hrs after CLP. In an additional experiment, all P-450 isoforms were inhibited by pretreatment with 1-aminobenzotriazole to determine the effect of cytochrome P-450 blockade on inflammatory responses by assessing proinflammatory cytokines. The results show further increases in serum concentrations of tumor necrosis factor-α, interleukin-1β, and interleukin-6 in aminobenzotriazole-treated animals at 10 hrs after CLP, which was associated with elevated concentrations of circulating lactate and severe morphologic alterations in the liver. These results suggest that the integrity of the cytochrome P-450 enzyme system plays an important role in septic inflammatory response. Conclusion: The major hepatic P-450 isoform CYP1A2 is down-regulated and inhibition of P-450 enzyme system is associated with an exacerbated inflammatory response in sepsis. Treatment with pharmaceutical agents that regulate or are metabolized by P-450 enzymes might be approached cautiously in the septic patient if this holds true in a clinical setting.