AbstractA capillary gas chromatographic method with nitrogen specific detection is presented for measuring pseudoephedrine and its major metabolite norpseudoephedrine in urine after derivatization with trifluoroacetic anhydride. After the oral intake of 49·2 mg pseudoephedrine (ACTIFED®) the active substance is nearly quantitatively excreted in urine over a 48 hr period. From 1 to 7 per cent is metabolized to norpseudoephedrine. The intake of acetazolamide results in a suppression of the pseudoephedrine concentration for at least 12h. The diuretic effect after the intake of 1·51 mineral water could be compared with the effect obtained with 1 mg bumetanide and results in a decrease in pseudoephedrine concentration by a factor 4 for several hou