A new extraction method and a new gas chromatographic (GC) assay for the determination of carbamazepine (CBZ) in human plasma are described. The use of a new internal standard, 2-methylcarbamazepine (MCBZ), was studied. The extraction method for CBZ was designed in such a manner so as to permit recovery from plasma of five other antiepileptic agents: phenobarbital (PB), phenytoin (PHT), primidone (PD), ethosuximide (ES), and 5-ethyl-5-phenylhydantoin (EPH). The first step involves the use of an ether extraction by which all six drugs and their appropriate internal standards are removed from the plasma sample. An on-column methylation technique (OCMT), which involves extraction of the ether phase with 0.3 M trimethylphenylammonium hydroxide (TMPAH), is carried out for the assay of PB, PHT, PD, ES, and EPH. CBZ and MCBZ, which remain in the ether phase when the extraction with TMPAH is carried out, are then isolated by a new, convenient extraction method. CBZ is determined by GC as its underivatized compound. Partition ratios (K‘) were determined for CBZ and MCBZ at each step of the extraction scheme to furnish the basis for the method. By use of the K’ values, the theoretical percentages of overall recoveries of CBZ and MCBZ were calculated to be 54 and 49%, respectively. The use of MCBZ as the internal standard in GC CBZ assays eliminates many of the problems associated with the thermal degradation of CBZ in GC.