We describe a method for the measurement of a metabolite of ortho-toluidine in urine. The method uses capillary gas chromatographic separation with mass spectrometric detection to quantitate the metabolite, and it requires no derivatization or extraction of the urine sample prior to analysis. Quantitation is accomplished by comparison with a control spiked with a standard of the metabolite. The coefficient of variation for day-to-day reproducibility of the assay was 9.7%. The limit of quantitation was 10 micrograms/L (10 parts per billion).