Abstract: To investigate the effects of carbamazepine (CBZ) on the plasma concentrations of valproic acid (VPA) and its toxic metabolite 2-propyl-4-pentenoic acid (4-ene VPA) in epileptic patients, the plasma concentrations of VPA and 4-ene VPA were determined, and the effect of CBZ on pharmacokinetics of VPA was evaluated. All patients had been divided into two groups (VPA group, n = 87; and VPA+CBZ group, n = 19). As compared to VPA group, the combination of CBZ significantly (P < 0.01) decreased the trough concentration of VPA VPA group, (69.5 ± 28.8) �g·mL^-1; VPA+CBZ group, (46.3 ± 25.6) �g·mL^-1 and does-adjusted VPA trough concentration VPA group, (4.89 ± 2.21) �g·mL^-1·mg^-1·kg^-1; VPA+CBZ group, (3.14 ± 1.74) �g·mL^-1·mg^-1·kg^-1. However, the addition of CBZ did not influence the concentration of 4-ene VPA. The present study revealed that coadministration of CBZ can reduce VPA plasma concentration and may impact VPA clinical effect, therefore therapeutic drug mornitoring of VPA should be used when combined use of CBZ and VPA.