1. After oral doses to dogs of14C-tinidazole, a 5-nitroimidazolyl antiprotozoal compound, a major and previously unidentified radioactive metabolite was isolated from urine and shown by FAB mass spectrometry and n.m.r. spectroscopy to be ring-hydroxylated.2. The exact identity of this metabolite was established by X-ray diffraction analysis as ethyl 2-(5-hydroxy-2-methyl-4-nitro-1-imidazolyl)ethyl sulphone.3. The apparent migration of the nitro group from the 5 position in the parent drug to the 4 position in the metabolite is a novel metabolic route.