A simple procedure for the simultaneous determination of carbamazepine and its major metabolite, carbamazepine epoxide, in plasma and urine is described. The assay involves two extractions of the drugs and an internal marker, clonazepam, from the alkalinized sample. The extract is evaporated to dryness at 45 degrees C and the residue is redissolved in methanol (30 microliters). A 25-microliters aliquot is injected into the liquid chromatograph and eluted with acetonitrile-water (40:60, v/v) on a C18 pre-column linked to a 5-microns C8 reversed-phase column. The eluent is detected at 215 nm. The method has been used to investigate the steady-state concentrations of carbamazepine and carbamazepine epoxide in the plasma and urine of a manic-depressive patient.