An isocratic liquid-chromatographic method employing one extraction step has been developed for the quantitation of five drugs and three metabolites in human plasma. The method uses 0.100-ml aliquots of human plasma and two internal standards. Chromatographic conditions include a 4.6 mm x 150 mm Spherisorb ODS2, 3 μm a high-performance liquid chromatography, (HPLC) column, a phosphate buffer-acetonitrile-methanol (700:160:140) mobile phase, and ultraviolet (UV) absorbance detection at 210 nm. Analytes and linear quantitation ranges μg/ml) were felbamate (FBM) 0.391–200; primidone (PRIM), 0.098–100; phenobarbital (PHENO), 0.195–100; carbamazepine (CBZ), 0.195–100; phenytoin (PHT), 0.195–200. For CBZ-transdiol (CBZ-TR) CBZ-epoxide (CBZ-EP), and the PHT metabolite, 5-(4-hy-droxyphenyl)-5-phenylhydantoin (HPPH), the range was 0.049–25.0 μg/ml. Ethosuximide, methsuximide, 2-methyl-2-phenyl-succinimide (methsuximide metabolite), 2-ethyl-2-phenyl malonamide (PRIM metabolite, 5-ethyl-5-(4-hy-droxyphenyl)-barbituric acid (PHENO metabolite), and mephenytoin do not interfere with quantitation of the above compounds.