Extractive alkylation was carried out on fenquizone, a sulphonamide diuretic, in order to devise a suitable method for its determination in pharmacokinetic and bioavailability studies. After extraction as a tetramethyl derivative, fenquizone was evaluated by gas-liquid chromatography with a 63Ni electron-capture detector, which enables a limit of detection of 2 ng/ml of plasma or urine to be achieved. Linearity was verified in a range of 50- 10,000 pg for each injection with a fenquizone/internal standard ratio ranging from 4:1 to 1:4. Determination is very rapid, as one analysis only takes 5 min. The preliminary results of the pharmacokinetic study performed in a volunteer human subject after a single oral administration of the drug are presented in this paper in terms of the plasma levels and the cumulative urinary excretion.