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Racemic 5-(4'-fluorophenyl)-5-phenylhydantoin was synthesized to examine its metabolism in rat. This compound differs from the antiepileptic agent 5,5-diphenylhydantoin in that the normal site of hydroxylation in 5,5-diphenylhydantoin is blocked on one of the phenyl groups by a fluorine atom. The 4'-fluoro analogue gives a major metabolite, which was isolated and identified as (R)-(-)-5-(4'-fluorophenyl)-5-(4'-hydroxyphenyl)hydantoin of 37% enantiomeric purity. The absolute configuration and enantiomeric purity of the metabolite was determined by chemical conversion to (S)-(-)-5-(4'-hydroxyphenyl)-5-phenylhydantoin. A second metabolite of the catechol type, possibly as a mixture of 5-(3',4'-dihydroxyphenyl)-5-(4'-fluorophenyl)hydantoin, and the corresponding O-3'-methyl derivative was detected by gas chromatography-mass spectrometry after methylation.