A column-switching system for the determination of the medium polarity diuretic acetazolamide in urine, has been designed. An Hypersil ODS C18, 30 microns (20 x 2.1 mm I.D.) pre-column was used for the pre-concentration and separation of acetazolamide from the biological matrix. The most polar urinary compounds were removed by washing the pre-column with a phosphate buffer solution (pH 3), and the fraction of eluate containing the analyte was switched to a LiChrospher RP C18, 5 microns (125 x 4 mm I.D.) analytical column, where it was chromatographed using gradient elution with acetonitrile-water, and detected at 275 nm. The most apolar urinary compounds were directly discarded by means of a second switching valve. Under these conditions the recovery of drug was 96 +/- 5% in the 0.50-100.0 micrograms/ml concentration range. The limit of detection was 10 ng/ml, the total analysis time being less than 8 min.