A coumarin mercapturic acid, N-acetyl-S-(3-coumarinyl)cysteine, has been identified in the urine of coumarin-treated rats. [14C]Coumarin was applied by gavage as a single dose to male Wistar rats (10-150 mg/kg body weight). Twenty-four-hour urine was collected, and the deproteinized concentrate was analyzed for radiolabeled metabolites by HPLC. The new mercapturic acid metabolite is supposed to result from oxidative biotransformation of coumarin to its 3,4-epoxide and subsequent coupling with glutathione.