1. 3,4,5,6-Tetrahydro-2H-azepine is an intermediate in the enzyme-catalyzed conversion of 1,6-diaminohexane to 6-aminohexanoic acid and its corresponding lactam, caprolactam, by mammalian liver aldehyde oxidase.2. Identification of metabolites was based on analysis by gas chromatography-mass spectrometry and confirmed by comparison with the properties of authentic standards.3. The results indicate that the cell differentiating agent hexamethylene bisacetamide is converted into 1,6-diaminohexane, and its metabolism therefore involves diamine oxidase.4. The metabolic fate of 1,6-diaminohexane is similar to that of putrescine and cadaverine in that a cyclic imine is an intermediate in the formation of metabolites with ring (lactam) and chain (amino acid) structures.