SINCE the discovery of chlorothiazide by Beyer et al. 1, a number of chemically related compounds have been shown to possess similar natriuretic and chloruretic actions2–4. Some of these agents, notably 3,4-dihydrochlorothiazide and its derivatives, differ from chlorothiazide primarily in a greater milligram potency, which may be associated with a longer duration of action5. These agents also inhibit carbonic anhydrase to a lesser extent than the unsaturated analogues, resulting in a lower urinary pH and generally a greater chloruretic response5,6.