The metabolite of 5-ethyl-5-(p-fluorobenzoylpropyl) barbituric acid (coded as F-B-1) in rabbit has been separated with HPLC. UV-spectra of F-B-1 and its metabolite obtained with DAD, in combination with EI-MS and CI-MS of F-B-1, its metabolite and their derivatives were examined. Possible structure of the metabolite was obtained. Possible metabolic way of F-B-1 is the reduction of carbonyl on the benzene ring with formation of hydroxyl group. This was further confirmed by identity of HPLC and MS behavior of the metabolite and NaBH4 reductive product. In the structure of the latter, the carbonyl on the benzene ring was reduced, as proved by NMR (1H-NMR and 13C-NMR) spectra.