A genetic deficiency in the metabolism of dextromethorphan and mephenytoin may be revealed by the excretion pattern of dextromethorphan and its metabolite dextrorphan, and mephenytoin, 4-OH-mephenytoin, respectively, after a single dose of the test drugs. Existing methods were modified for determining the compounds in 0.1-0.5 ml urine samples. No prior derivatization of the compounds was necessary before their gaschromatographic or mass-spectrometric analysis by using crosslinked 5% phenylmethyl silicone fused silica columns. Seven healthy volunteers were phenotyped at weekly intervals with either 25 mg dextromethorphan or 100 mg mephenytoin, or both drugs. One subject was a poor metabolizer of mephenytoin, while all subjects were extensive metabolizers of dextromethorphan. Neither a pharmacokinetic nor an analytical interference was observed when the results of the single test were compared with those of the combined test. The results of the mephenytoin test were also tentatively given in form of metabolic ratios. The GC-MS assay was designed for clinical studies so that patients treated with other drugs could be phenotyped.