Abstract: A simple, sensitive and reproducible HPLC assay is described for the determination of mephenytoin and 4'-hydroxymephenytoin in human urine. Phenobarbital was used as an internal standard.The compounds were separated on a U-Bondapack RP-C₁₈ column using a mobile phase of and the UV detectou was set at 210 nm. Calibration curves in the range 0.05～1.00ug/ml for mephenytoin and 0.5～100.0ug/ml for 4'-hydroxymephenytoin were linear (r=0.9998and r=0.9992,respectivesy). The average recovery was 95.10±2。95%,and the relative standard devia- tion within day and day to day was less than 10%。The detection limit for mephenytoin was 25mg/ml and 4‘-hydroxymephenytoin was 50mg。ml。The method was used to study the metabolism of S-mephenytoin 4'-hydroxylatoin in 10healthy volunteers.The 12 h urinary metabolic ratio (MR)and hydroxylation index(HI)were calculated to express interindividual variation in metabolism. Two of them exhibited defective 4'-hydroxylation of S-mephenytoin as poor metabllizers (HI:1349.18 and 409.57;MR:105.29 and 8.25).In the remaining 8 subjects, the ranged from 1.68 to 6.71 and the MR ranged from 0.002 to 0.014,as extensive metabolizers of S-mephenytoin.