Chlorothiazide is a new orally given diuretic compound recently synthesized and shown to be a carbonic anhydrase inhibitor in vitro 1/25 as potent as acetazolamide.1Upon comparing the structural formulas of the two compounds (fig. 1), it may be seen that they possess the SO2NH2grouping in common and are thus both representative of that group of heterocyclic sulfonamides which has previously been shown to exert inhibition of carbonic anhydrase activity.2Experimental and clinical studies on the mechanism of the diuretic action of chlorothiazide have indicated that the effect of the drug is predominantly saluretic (leading to an increase in sodium and chloride excretion) and, to a lesser extent, kaluretic (leading to an increase in potassium excretion) and that it has minor but definite carbonic anhydrase-inhibiting effects.1Chlorothiazide has been described as the most potent orally given diuretic now available, having an approximate potency