A quantitative method was developed for valproic acid and five of its mono-unsaturated metabolites using capillary gas chromatography-mass spectrometry with selected ion monitoring. The method was applied to serum and all metabolites were measured in a single run. Methyl esters were synthesized as the derivatives suitable for gas chromatography. Calibration curves were found to be linear and the sensitivities in the order of 0.1 micrograms ml-1. Patients' data are presented. By this method it is possible to separate the stereoisomers of 2-n-propyl-2-pentenoic acid and of 2-n-propyl-3-pentenoic acid.