1. The metabolism of malotilate, possessing a 1,3-dithiole ring, was studied in rats. A major biliary metabolite of malotilate was isolated and determined to be a thio-glucuronide of the dithiol formed by ring-opening, namely, 1-mercapto-2,2-di(isopropoxycarbonyl)-ethenyl 1-thio-beta-D-glucosiduronic acid. The structure was elucidated by proton-n.m.r., 13C-n.m.r. and high-resolution mass spectrometry. 2. The thio-glucuronide was completely hydrolysed by beta-glucuronidase. This enzymic reaction was inhibited by saccharo-1,4-lactone.