Urine samples from three children at different stages of chronic valproate therapy were partially purified using a cation exchange column. A signal consistent with either valproylcarnitine or octanoylcamitine was observed in one of these extracts by direct fast atom bombardment-mass spectrometry analysis. These isomeric acylcarnitines were synthesized, separated and characterized by thermospray high performance liquid chromatography-mass spectrometry. This new technique was then employed to positively identify intact valproyl-camitine in the patients' urine samples. The implications of this finding with regard to a mechanism to account for carnitine deficiency in patients receiving valproate are discussed.