The reactive metabolite S-naproxen-beta -1-O-acyl glucuronide was purified from human urine using solid phase extraction (SPE) and preparative HPLC. The structure was confirmed by 600 MHz H-1 NMR. Directly coupled 600 MHz HPLC-H-1 NMR was used to assign the peaks in chromatograms obtained when analysing a sample containing S-naproxen aglycone and the 1-, 2-, 3-, and 4-isomers of S-naproxen-beta -1-O-acyl glucuronide in two simple isocratic reversed phase HPLC-systems. Using mobile phase I (50 mM formate buffer pH 5.75/acetonitrile 75:25 v/v) the elution order was: 4-O-acyl isomers, beta -1-O-acyl glucuronide, 3-O-acyl isomers, 2-O-acyl isomers, and S-naproxen aglycone. Using mobile phase II (25 mM potassium phosphate pH 7.40/acetonitrile 80:20 v/v) the elution order was: alpha/beta -4-O-acyl isomers, S-naproxen aglycone, beta -1-O-acyl glucuronide, 3-O-acyl isomers, and alpha/beta -2-O-acyl isomers. In both systems the elution order for the 2-, 3- and 4-O-acyl isomers corresponded with previously published results for 2-, 3-, and 4-fluorobenzoic acid glucuronide isomers determined by reversed phase HPLC-H-1 NMR [U.G. Sidelmann, S.H. Hansen, C. Gavaghan, A.W. Nicholls, H.A.J. Carless, J.C. Lindon, I.D. Wilson, J.K. Nicholson, J. Chromatogr. B Biomed. Appl. 685 (1996) 113-122]. The alpha -1-O-acyl isomer was found to be present at approximately 3% of the initial S-naproxen-beta -1-O-acyl glucuronide concentration in the glucuronide isomer mixture after 6 h of incubation at pH 7.40 and 37 degreesC. In both HPLC systems it eluted just before the beta -1-O-acyl glucuronide well separated from other isomers. Investigators should consider the possible formation of a alpha -1-O-acyl isomer when studying,glucuronide reactivity and degradation. (C) 2001 Elsevier Science B.V. All rights reserved.