To evaluate the effects of valproic acid on the disposition of phenytoin, a single dose of 600 mg valproic acid and multiple doses of valproic acid (200 mg four times a day for 5 days) were administered together with a single oral dose of 600 mg phenytoin to 12 young male volunteers. Fraction of unbound phenytoin and the area under curve (AUC) of the total and unbound phenytoin in plasma were compared with the control phase in which only 600 mg phenytoin was given. Valproic acid increased the unbound fraction of phenytoin in both single- and multiple-dose studies by 15 per cent and 41 per cent, respectively. Single-dose valproic acid increased the total AUC of phenytoin by 11 per cent. Multiple-dose valproic acid decreased the total AUC by 7 per cent. Single- and multiple-dose valproic acid increased the unbound AUC by 25 per cent and 18 per cent, respectively, probably due to the inhibition on the metabolizing enzymes. We concluded that there are at least two mechanisms involved in valproic acid-phenytoin interaction. Whereas valproic acid displacing phenytoin on the plasma protein decreased the total drug concentration of phenytoin, the enzyme inhibition by valproic acid increased both the total and unbound concentration of phenytoin. The two conflicting mechanisms may result in different effects on the total plasma concentration of phenytoin. Therapeutic drug monitoring based on the total concentration of phenytoin may be misleading when valproic acid is co-administered.