A gaschromatographic procedure for the simultaneous determination of ethosuximide and valproate is presented. After addition of the two internal standards the serum sample is acidified and extracted by chloroform. 2 microliters of the organic phase is directly injected into the gaschromatograph without previous evaporation. The gaschromatographic analysis is carried out on SP 1000 under isothermal conditions without derivatisation. The coefficients of variation for the precision from day to day were 2.6% (ethosuximide) and 5.2% (valproate). In external quality control the results for both substances always differed less than one standard deviation from the mean. The detection limits (11.6 mumol/l for ethosuximide and 2.3 mumol/l for valproate) were far below the usual therapeutic range. The specificity of the method was proven by comparison of the relative retention times of about 100 drugs, the investigated antiepileptic drugs and their internal standards.