Urine of rats treated with thiophene contains a very major metabolite which represents about 30% of the administered dose. A detailed analysis of its 1H and 13C NMR spectra and a study of its IR and mass spectra clearly showed that it was a 2,5-dihydrothiophene sulfoxide bearing a N-acetyl-cysteinyl group on position 2. Upon heating, it lost water with formation of N-acetyl-S-(2-thienyl)-L-cysteine. A likely mechanism for the formation of this metabolite should involve the S-oxidation of thiophene as a primary step and the addition of glutathione to the very reactive thiophene-S-oxide. These data provide a first evidence for the intermediate formation in vivo of thiophene-S-oxides as reactive metabolites.