Diuretics may be classified according to their chemical structure, their mechanism and site of action within the nephron, and their diuretic potency. Those agents with primary action in the proximal nephron include the carbonic anhydrase inhibitors, e.g. acetazolamide, a sulfonamide derivative. Other drugs containing the sulfonamido grouping, e.g. furosemide, chlorothiazide and metolazone, also have secondary effects on the proximal nephron. Those drugs which have their major pharmacologic activity within the ascending limb of the loop of Henle, inhibiting the sodium/potassium/2 chloride electroneutral transport system, include the sulfonamide agents furosemide, bumetanide, piretanide and torasemide, and the phenoxyacetic acid derivative, ethacrynic acid. In the early portion of the distal convoluted tubule, sodium chloride reabsorption is impaired by the thiazide group, indapamide and metolazone, as their primary site of action. In the late reaches of the distal convolution and in the collecting duct, agents that inhibit the exchange of sodium for that of hydrogen and potassium have their major sites of activity. These agents, spironolactone, amiloride and triamterene, differ not only chemically but in their mechanisms of action. Diuretics may also be grouped according to potency. The loop of Henle agents are the most powerful, causing the excretion of 20-25% of filtered sodium load. The thiazide group and metolazone are moderately potent, resulting in the excretion of 5-8% of filtered sodium, and the ‘potassium-sparing’ drugs are only mildly potent, causing the excretion of only 2-3% of filtered sodium.