Naproxen is metabolized in humans by O-demethylation, and by acyl glucuronidation to the 1-O-glucuronide. Naproxen, its metabolite and the conjugates can be measured directly by gradient high-performance liquid chromatographic analysis without enzymic deglucuronidation. The glucuronide conjugates were isolated by preparative chromatography from human urine samples. Mild acidic hydrolysis of one urinary conjugate resulted in naproxen. This conjugate was also formed by alkaline isomerization of isolated naproxen acyl glucuronide, indicating that the structure of this urinary conjugate must have been naproxen isoglucuronide (4-O-glucuronide). Mild acidic hydrolysis of another urinary conjugate resulted in O-desmethylnaproxen. This conjugate was also formed by alkaline isomerisation of isolated O-desmethylnaproxen acyl glucuronide, indicating that the structure of this urinary conjugate must have been O-desmethylnaproxen isoglucuronide (4-O-glucuronide). Calibriation curves were constructed by enzymic deconjugation of samples containing different concentrations of isolated naproxen acyl glucuronide, O-desmethylnaproxen acyl glucuronide, and the isoglucuronides of naproxen and O-desmethylnaproxen by mild acidic hydrolysis. The limit of quantitation of naproxen in plasma is 1.5 microgram/ml. The limits of quantitation in urine are: naproxen, O-desmethylnaproxen, naproxen acyl glucuronide and O-desmethylnaproxen acyl glucuronide, 1 microgram/ml; the isoglucuronide of naproxen and O-desmethylnaproxen, 1.5 microgram/ml. A pharmacokinetic profile of naproxen is shown, and some preliminary pharmacokinetic parameters of naproxen obtained from two human volunteers are given.