Four volunteer subjects were exposed to 150ppm (655 mg/m3) of ethylbenzene and 150ppm (655 mg/m3) of m-xylene both separately and in combination. The biotransformation of the solvents was studied on the basis of the metabolites found in the urine. The metabolic conversion of both m-xylene and ethylbenzene proceeded mainly through oxidation of side chains. Ring oxidation seemed to be of minor importance; in the case of ethylbenzene it accounted for 4.0% (combined share of 4-ethylphenol, p- and m-hydroxyacetophenones) and in case of m-xylene for 2.5% (2,4-dimethylphenol), respectively. Mandelic and phenylglyoxylic acids amounted to 90% of the ethylbenzene metabolites, whereas m-xylene were excreted to 97% in the form of m-methylhippuric acid. Almost equimolar amounts in the form of metabolites of both solvents were found in the urine during 24h from the onset of exposure. Most of the ethylbenzene metabolites were excreted at substantially slower rates than those of m-xylene. The combined exposure resulted in a mutual inhibition of the metabolism of ethylbenzene and m-xylene, which was demonstrated by delayed excretion and decreased amounts of metabolites excreted. No sign of alteration in the urinary metabolite patterns of either ethylbenzene or m-xylene could be detected.