The structures of six metabolites of tenoxicam in rats (2 mg/kg, orally), elucidated by physicochemical analyses or the reverse-isotope dilution method, were 5'-hydroxytenoxicam (5% dose), 3-(methylsulphamoyl)-2-thiophenecarboxylic acid (9% dose), and the C-7 or C-8 O-glucuronide of tenoxicam (30% dose). The mechanism of formation of N-methylthiophenesulphimide, a possible precursor of 3-(methylsulphamoyl)-2-thiophenecarboxylic acid from tenoxicam, is discussed.