A simple sensitive method of high specificity and selectivity for quantitative determination of the non-steroidal anti-inflammatory drug naproxen and its main metabolite, 6-demethylated derivative, in biological specimens is described. Like naproxen, its metabolite absorbs maximally at 232 om; this makes their simultaneous quantification, via direct UV-measurements at λmax, in biological fluids quite impossible. Simple TLC-separation on silica gel F254 using chloroform + methanol (85:15, v/v) achieved the best fractionation of the unchanged drug and its metabolite from the matrix-contents of urine. UV-quantification of fractionated components could reach concentration levels of 0.2–3.0μg ml−1 (ppm) in worked up urine samples. Varying levels of unchanged antiinflammatory drug and the phenolic metabolite could be accurately traced in urine samples following a 2.9 mglk.g oral dose after different time-intervals. Synthetic preparation of the metabolite by demethylation of naproxen is briefly mentioned.