toxic. After the discovery of the car­ bonic anhydrase, in the early 1930s, and the introduction of sulfanilamide as a chemotherapeutic agent, it was observed that this drug was inhibiting carbonic anhydrase in vitro and urinary acidification in vivo thereby causing metabolic acidosis; urine output, however, appeared to increase. Subsequent studies led to the synthesis of more potent analogs, in particular acetazolamide. Studies on car­ bonic anhydrase inhibitors led to the synthesis of benzothiadiazides which disclosed much less inactivating action on carbonic anhydrase and much more diuretic effect through an inhibition of tubular transport of sodium and chloride. Chlorothiazide was the first member of this class of diuretics. Thiazides are still used in clinical practice.