A method for the analysis of valproic acid in plasma or serum has been developed. The specimen is deproteinized with acetonitrile containing nonanoic acid as internal standard and an aliquot of supernate is heated at 70 degrees C for 15 min with 4-bromophenacyl bromide at pH 7. An aliquot of the reaction mixture is subjected to reverse phase high performance liquid chromatography at ambient temperature and a detector wavelength of 254 nm. Drug and internal standard eluted within 6 min. The method correlated well with a gas chromatographic procedure. Recoveries ranged from 85 to 108% and the day-to-day coefficients of variation ranged from 3.8 to 10.7%.