Reduction in the blood free carnitine (FC) level as a side effect of sodium valproate (VPA) given epileptic patients was pharmacokinetically studied in connection with changes in the VPA disposition. The serum FC level in patients taking at least one of phenobarbital (PB), phenytoin (PHT) and/or carbamazepine (CBZ) in addition to VPA was significantly lower than that in the controls given only these other anti-epileptic drugs (AEDs). Patients medicated only with VPA also tended to have a lower serum FC level than the controls, although the difference was not significant. Among all the patients taking VPA with or without other AED(s), a significantly positive correlation was observed between the serum FC level and the value of dose and level ratio (L/D) of VPA, indicating that both the serum FC concentration and the L/D value of VPA were remarkably reduced in those patients receiving both medications. These results suggested that reduction in the blood FC level as a side effect of VPA reflected FC deficiency associated with the accelerated degradation of VPA in liver; such a condition appears to result from medication with VPA and other AED(s) which induce(s) enzyme(s) for the VPA metabolism.