A stereospecific assay for the simultaneous determination of the obtained by an MS method, involving administration of a synthetic pseude enantiomers of warfarin and its major metabolites, 6-and 7-hydroxywarfarin racemate ['*C(R),'T(S)]warfarin. In addition to all known metabolites, and warfarin alcohols, in plasma and urine was developed. Involved in this the detection of 7-R-hydroxywarfarin indicates that 7-hydroxylation is stedetermination was the formation of diastereoisomeric esters with carboben-reoselective rather than stereospecific. zyloxy-L-proline. separation by normal-phase high-performance liquid chromatography, and detection by fluorescence after postcolumn aminolysis KeypbrPses F1uorescencel HPLC-stereospecific Of warfarin and with n-butylamine. The determination limit for any enantiomer is in the order its of 50-100 ng. The method was applied to the analysis of the enantiomers of 0 Isomers-stereospecific fluorescence HPLC. warfarin and its metabolites warfarin and metabolites in plasma and urine of human subjects receiving racemic drug. The results for warfarin enantiomers are comparable with those Warfarin-stereospecific fluorescence HPLC* Warfarin [3-(a-acetonylbenzyl)-4-hydroxycoumarin] contains an asymmetric center. In humans, the Rand S-isomers show differences in pharmacological activity (I), hepatic clearance, and metabolism (2-4). Drugs have been shown to interact differently with the isomers; for example, when coadministered with phenylbutazone, the hepatic clearance