N-(5-nitro-2-thiazolyl)-N'-carboxymethylurea (NTCU) has been identified as a urinary metabolite of the antischistosomal drug niridazole [1-(5-nitro-2-thiazolyl)-2-imidazolidinone]. When DBA/2J mice were treated with [14C]niridazole, a metabolite comprising 12-14% of the total radioactivity in 24-hr urine samples was resolved by HPLC. The compound was subsequently isolated from pooled urine of niridazole-treated patients. It was identified as NTCU by mass spectrometry, and the deduced structure was confirmed by chemical synthesis. NTCU is unique among known niridazole metabolites, because it lacks an intact imidazolidinone ring. Its structure allows for a ketoenol tautomerism in which the enolate is stabilized by conjugation with the nitrothiazole ring, as evidenced by a pH-dependent 80-nm red shift in the absorption spectrum. We hypothesized that NTCU arises via oxidation of an acyclic aldehyde tautomer of 5-hydroxyniridazole, one of two proximate oxidative niridazole metabolites. Indirect evidence for the aldehyde tautomer included the fact that 5-hydroxyniridazole displayed the same pH-dependent spectral shift as NTCU with a single isobestic point at 388 nm. The proposed precursor-product relationship was confirmed when we found that NTCU formation from 5-hydroxyniridazole was catalyzed by NAD(+)-dependent aldehyde dehydrogenase (EC 1.2.1.3). The activity copurified with benzaldehyde dehydrogenase activity from mouse liver cytosol. Furthermore, benzaldehyde was a competitive inhibitor of 5-hydroxyniridazole dehydrogenase activity. These results demonstrate that 5hydroxyniridazole is not an end product of niridazole metabolism. Because biotransformation of niridazole to its 4- and 5-hydroxy derivatives has been implicated in the drug's carcinogenicity and central nervous system toxicity, NTCU formation appears to represent a detoxication pathway in mammals.