Abstract: A gas chromatographic method equipped with nitrogen-phosphorus detector was developed for the determination of the S-and R-enantiomers of the anticonvulsant, mephenytoin (MP) in human urine. Dichioromethane (4 ml) was added to 1 ml urine, the mixture was shaken and centrifuged. The organic phase was transferred to another tube and blown to dryness under nitrogen on water bath (37℃). The residue was dissolved in 10 μl ethylacetate and 1～2 μl was injected into the GC. Our results showed that direct enantiomeric separation of mephenytoin was obtained by using a chiral capillary column, the retention times for S-and R-mephenytoin were 25. 5 and 26. 2 min respectively, with a detection limit less than 50 ng/ml of mephenytoin. Similar linear and reproducible standard curves were obtained over the concentration range of 53.2 to 2128. 0 ng/ml (for S-MP, r=0. 9914±0. 0070, n=6; and for R-MP, r=0. 9939±0. 0070, n=6), and the mean recoveries of S-and R-MP were 95. 4% and 95. 8% respectively. The within--day relative standard deviations were less than 8. 8% for both S-and R-MP, and that of between--days were less than 14.3%. There was a good reproducibility of the urine S/R mephenytoin determined in China and in Sweden by using similar method in 107 Chinese volunteers after a single oral dose of 100 mg racemic mephenytoin (r=0. 9091, P<0. 001).