Lidocaine (xylocaine) is utilized for the treatment of ventricular arrhythmias which occur during cardiac surgery or myocardial infarction and as a local anesthetic. Recent data from the National Toxicology Program reported that a principal metabolite in man, 2,6-dimethylaniline, is carcinogenic in rats. In addition, the putative metabolite N-hydroxy-2,6-dimethylaniline has been reported to be mutagenic in Salmonella typhimurium TA100. N-Hydroxy metabolites of aromatic amines may be oxidized by hemoglobin to the corresponding nitroso metabolites and the nitroso may covalently bind to cysteine groups in hemoglobin as the corresponding sulfinic acid amide. Since hemoglobin binding is an indirect measure of the formation of the N-hydroxy metabolite, we have examined the possibility that lidocaine or a metabolite may similarly covalently bind to hemoglobin in rats and humans. Using a previously developed gas chromatographic-mas spectrometric assay, hemoglobin adducts of 2,6-dimethylaniline were detected covalently bound to rat hemoglobin after administration of either 2,6-dimethylaniline or lidocaine. Consistent with previously reported observations, low levels of 2,6-dimethylaniline-hemoglobin adducts were also observed in human subjects before lidocaine administration. Following administration of lidocaine, all patients had much higher levels of 2,6-dimethylaniline-hemoglobin adducts. Differences in adduct levels in patients treated with lidocaine (70-3760 mg) ranged from 93 to 636 ng/g hemoglobin. These data indicate that N-hydroxy-2,6-dimethylaniline is a metabolite of lidocaine in man.