Abstract: The metabolite of 5-ethyl-5-( p-fluorobenzoylpropy)harbituric acid(coded asF-B-1) in rabbit has been separated with HPLC. UV-spectra of F-B-1 and its metabolite obtained withDAD, in combination with EI-MS and CI-MS of F-B-1, its metabolite and their derivatives wereexamined. Possible structure of the metabolite was obtained.Possible metabolic way of F-B-1 is thereduction of carbonyl on the benzene ring with formation of hydroxyl group. This was furtherconfirmed by identity of HPLC and MS behavior of the metabolite and NaBH4 reductive preduct. Inthe structure of the latter, the carbonyl on the benzene ring was reduced, as proved by NMR(lH-NMR and ¹³C-NMR)spectra.