The chemical structure of an ethimizol metabolite found earlier in human urine and saliva was established as 4,5-dicarbamoyl-1-ethylimidazole (didesmethylethimizol). The treatment of biological samples comprised a solid-phase extraction, high-performance thin-layer chromatographic separation and high-performance liquid chromatographic purification techniques. The identification of the metabolite was based on the comparison of a mass spectrum of the substance isolated from the biological fluid with the mass spectra of synthetised reference compounds.