A single high-recovery extraction process is described for rapid gas-liquid chromatographic (GLC) determination of valproic acid (VPA). The drug is extracted from acidified plasma into an equal volume (0.2 ml) of chloroform. Evaporation is avoided by extraction into a chloroform bed, from which an aliquot is analysed by isothermal GLC employing a column packed with 10% DEGS. The reproducibility and accuracy of the method was good. In 100 plasma samples from patients, there was an excellent correlation (r= 0.98) between VPA levels determined concomitantly by this method and the technique of Kupferberg. Single plasma samples can be analysed in 20 min, and 40 samples can easily be analysed daily by a single technician. Ethosuximide concentrations can also be determined simultaneously. The method is suitable for emergency and routine use in drug monitoring laboratories.