Concentrations of carbamazepine (CBZ) and its metabolite carbamazepine-10, 11-epoxide (CE) were determined in serum (free and total), brain tumors and paratumorous cortex from 37 epileptic patients. Tumor biopsies were available from 25 different gliomas, 5 hamartomas and 1 angioma. The free serum concentrations of CBZ and CE were highly correlated with the corresponding paratumorous concentrations (r = 0.94 and r = 0.93) and to a lower extent with the tumorous concentrations (r = 0.78 and r = 0.75). For CBZ the concentration ratios paratumorous cortex/serum free and tumor/serum free showed no significant differences between CBZ-resistant and -nonresistant patients. Apart from histological grade a significant influence of type and site of the tumors on their CBZ and CE concentration was not recognizable. The results indicate that resistance to CBZ therapy cannot be sufficiently explained by decreased paratumorous or tumorous levels of CBZ and CE.