1. It has recently been proposed that acyl coenzyme A thioesters (acyl-CoAs) of xenobiotic carboxylic acids are electrophilic, reactive metabolites that may react with proteins. 2. The primary objective was to investigate the reactivity of the tolmetin acyl coenzyme A thioester (Tol-CoA). The second objective was to identify and quantify tolmetin (Tol) metabolites in vivo that were formed via Tol-CoA, e.g. the glycine (Tol-Gly) and taurine (Tol-Tau) conjugates. This finding would be indicative of Tol-CoA formation and thus of other acyl-CoA-related reactions that might occur, e.g. covalent binding to proteins. 3. In order to study the chemical reactivity, Tol-CoA (0.5 mM) was incubated with glutathione (5 mM) in a 0.1 M phosphate buffer (pH 7.4) at 37 degrees C. Tol-CoA reacted rapidly with glutathione in vitro to form the S-acyl glutathione conjugate at a rate of 14.9 +/- 0.7 micro M min(-1) (mean +/- SD, n = 3) from 0 to 10 min. Compared with acyl-CoAs of other xenobiotic carboxylic acids, naproxen and clofibric acid, the rate by which Tol-CoA reacted with glutathione was high. 4. Following administration of (3)H-Tol (100 mg kg(-1), 200 micro Ci kg(-1), p.o.) to male Sprague-Dawley rats, Tol-Tau and Tol-Gly were identified in urine by electrospray ionization MS-MS in both positive- and negative-ion modes. The conjugates were only formed at trace levels (< 0.5%). However, the presence of Tol-Tau and Tol-Gly showed the reactive Tol-CoA was formed in vivo.