Simultaneous steady-state plasma concentrations of carbamazepine and carbamazepine-10,11-epoxide have been measured by high pressure liquid chromatography in 101 epileptic children and adults taking the drug. There was either no statistically significant correlation, or only a very poor correlation, between drug dose and steady-state plasma levels of a) carbamazepine, b) its epoxide, and c) the sum of drug and epoxide. Plasma concentrations of carbamazepine correlated with those of it epoxide. Plasma carbamazepine levels were lower in patients taking phenytoin with carbamazepine than in patients taking carbamazepine alone. Plasma carbamazepine-10,11-epoxide levels were not definitely altered when carbamazepine and phenytoin were used together. This finding is consistent with the hypothesis that phenytoin enhances the metabolism of carbamazepine to a metabolite other than its epoxide.