The effects of hyperglycemia on the pharmacokinetics of valproic acid (VPA) were examined by time-concentration profiles of plasma VPA accompanied with blood glucose (BG) changing. In addition, time-concentration profiles of plasma free fatty acids (FFAs) were also obtained to examine the interaction between VPA and FFAs in vivo. For the experiments in vivo, normal rats, given multiple doses of maltose orally, and diabetic rats, which were made to maintain hyperglycemia, were used. Plasma VPA and FFA were determined by high-performance liquid chromatography with electrochemical detection (HPLC-ECD) systems based on the reduction of quinone for the selective determination of acids, respectively. BG was determined by pocket-size glucose meter. The maximum plasma concentrations (Cmax) of VPA in normal rats given multiple doses of maltose orally and in diabetic rats were remarkably decreased in comparison with those in the control rats. From the present study, it was shown that the metabolism of plasma VPA is accelerated under hyperglycemia. Moreover, we also found that VPA was preferentially metabolized in comparison with the plasma FFA in vivo.