The determination of urinary o-cresol in varying exposure to toluene vapor showed a linear relationship between the metabolite and the inhaled solvent. The analytical method involves acid hydrolysis of 20 ml of urine with subsequent extraction of the phenolic compounds in dichloromethane, followed by gas chromatographic analysis. The lowest limit of detection lies at 0.5 micromol/l, whereas the practical limit as an exposure test is at 2.5 micromol/l [corresponding to 0.2 micromol/l (5 ppm) of toluene vapor]. The test may also have indirect toxicologic significance, as the underlying initial arene oxides which produce the cresolic compounds are involved in toluene toxicity.