A gas chromatographic method has been developed for the routine monitoring of valproic acid (VPA) in human plasma samples. Two compounds, 2-ethylpentanoic acid (EPA) and 2-propylhexanoic acid (PHA), were synthesized and evaluated as internal standards together with cyclohexane carboxylic acid (CHCA), a commonly employed internal standard. Crystalline barium salts of VPA, EPA, and PHA were prepared, which enabled preparation of standard solutions of high accuracy for use in calibration experiments and in daily intra-laboratory quality control tests. The extraction scheme was designed on the basis of the solvent partitioning properties of VPA. Solvent transfers are required in the extraction scheme, but solvent evaporations are not. Studies were made of the performances of EPA, PHA, and CHCA as internal standards in the VPA assay at different lifetimes of the 10% SP-1000 chromatography column. As judged by these studies, EPA or CHCA is a better choice than PHA as an internal standard, provided that certain guidelines are followed in the use of CHCA.