Fenoprofen (FPF) is a chiral non-steroid antiinflammatory drug, marketed as a racemic mixture of its R(-) and S(+) enantiomers. Its stereoselective disposition in humans and animals is due to a chiral inversion converting R(-)FPF into S(+)FPF. The first step of this reaction, which produces an acyl-CoA thioester, is catalysed by an acyl-CoA ligase. A stereospecific high performance liquid chromatography assay was used to study the disposition of FPF enantiomers in four geldings and three male beagle dogs, following intravenous doses of racemic FPF (1 mg/kg in horses), R(-)FPF (0.5 mg/kg in horses, 1 mg/kg in dogs), and S(+)FPF (0.5 mg/kg in horses, 1 mg/kg in dogs). A unidirectional stereoinversion of the R(-) enantiomer into its optical antipode (38% in horses, 90% in dogs) was demonstrated. This explained the clear enantioselective behaviour of FPF in both species. Acyl-CoA ligase activity (Km = 473.2 +/- 92.5 microM; Vmax = 23 +/- 3.3 nmol/min/mg) has also been quantified in vitro on equine hepatic microsomes, using a high performance liquid chromatography method to measure thioester formation. The present study showed that, in horses and dogs, as previously demonstrated in rats and sheep, the R(-)FPF clearance was better correlated with ligase activity than with inversion rate. A highly significant linear relationship was demonstrated between these variables.