Carbamazepine (CBZ) used in the treatment of epilepsy, is metabolized in man to an active metabolite: carbamazepine 10-11 epoxide (CBZ-E). CBZ and CBZ-E concentrations in the different blood compartments (plasma ultrafiltrate, plasma proteins, and red blood cells (RBC)) of epileptic patients on CBZ monotherapy were assessed using HPLC. The highest CBZ and CBZ-E level to dose ratios were observed in the RBC. For CBZ and CBZ-E significant correlations were found between some blood compartments particularly between RBC and plasma ultrafiltrate. The measurement of CBZ and CBZ-E in all blood compartments may be interesting in uncontrolled patients and in patients presenting side-effects which cannot be explained by total plasma concentration values.