ICI 207,828 is a novel eukalemic diuretic in animals that is comparable in effect to hydrochlorothiazide. We used micropuncture and microperfusion techniques to determine the site(s) of action of this compound in the rat nephron. Either furosemide (FUR) or ICI 207,828 were perfused through the loop of Henle in situ. Both compounds caused significant reduction in water and electrolyte reabsorption by the loop. The effect of ICI 207,828 was significantly less than that of FUR. Both amiloride and ICI 207,828 were perfused, in situ, through the superficial distal tubule. ICI 207,828 had an effect similar to amiloride. Sodium and water reabsorption and potassium secretion were inhibited. In free-flow micropuncture studies, ICI 207,828, infused continuously i.v., had little effect on electrolyte and water reabsorption by the superficial proximal tubule. This compound significantly inhibited water and electrolyte reabsorption by the loop of Henle. Distal tubule secretion of potassium was inhibited. In addition, fractional potassium reabsorption beyond the superficial, late distal tubule was inhibited by ICI 207,828. From these results, we conclude that ICI 207,828 significantly inhibits electrolyte and water transport by the loop of Henle and distal tubule, and at sites beyond the superficial distal tubule such as either the collecting duct system or juxtamedullary nephrons. The reduction in distal potassium secretion, in concert with reduced loop of Henle and postdistal reabsorption, results in no net potassium loss in the urine, thus rendering this compound eukalemic.