We previously demonstrated that the expression levels of drug-metabolizing enzymes, cytochrome P450 (CYP) enzymes, in the liver are significantly decreased in a murine model of ulcerative colitis (UC). In this study, we investigated changes in the pharmacokinetics of phenytoin, a CYP2C substrate drug, in the presence of UC. Colitis was induced by feeding male mice 3.5% dextran sulfate sodium (DSS) dissolved in drinking water for 10 days. The messenger RNA (mRNA) expression of CYP2C29 and CYP2C37 and the protein expression of CYP2C in the liver were evaluated via real-time reverse transcription-polymerase chain reaction and Western blotting, respectively. In DSS-treated animals, both mRNA and protein expression levels of CYP2C in the liver were significantly reduced relative to those in control animals (by 20%-40%). Phenytoin (30 mg/kg) was administered orally in a single dose to mice, and plasma concentrations were measured. Plasma concentrations of phenytoin were higher in the DSS-treated group than in the control group at 12, 24, and 36 hours after administration. Animals given DSS also exhibited a higher area under the plasma concentration-time curve extrapolated to infinity (AUC, 315 μg·h/mL), a delayed elimination half-life ( T, 8.1 hours), and a decreased body clearance (CL/F, 3.52 mL/h) compared with that of control animals (AUC, 215 μg·h/mL; T, 3.6 h; CL/F, 5.58 mL/h). This study indicated that the presence of UC decreases CYP2C expression levels in the liver, thereby delaying the metabolism of CYP2C substrates, including phenytoin, and increasing blood concentrations of these substrates.