Five patients with the clinical patterns of Bartter’s syndrome underwent a series of clearance studies in order to characterize the underlying tubule defect. Free water generation during maximal water diuresis (Ch₂o), expressed as percentage of the distal delivery (Ch₂o + C_cl), was lower in the patients (72.5 ± 3.2%) than in controls (84.4 ± 5.5, p < 0.0001). During maximal water diuresis and furosemide administration (40 mg i.v. as bolus), NaCl reabsorption along the diluting nephron segments could be separated into 2 components, that occurring in the loop of Henle (DR_NaHL) and that occurring in tubule segments beyond the macula densa (DR_NaDT): DR_nahl was normal, while DR_NaDT was reduced (3.1 ± 0.8 vs. 6.2 ± 2.5 ml/min in controls, p < 0.015). Thus, according to this furosemide protocol, our patients had normal solute reabsorption in the loop of Henle but reduced NaCl reabsorption in tubule segments beyond the macula densa. During 0.9% saline infusion (2 liters in 2 h, after stimulation of distal Na reabsorption with fludrocortisone) fractional excretion (FE) of K showed a linear rise with the increase of FE_cI.FE_K, however, was much higher in the patients than in controls for every FEci level. In contrast, the infusion of Na₂SO₄, after fludrocortisone administration, induced similar FE_K increases in patients and in controls. Thus, in these patients Na reabsorption in the distal nephron (possibly the cortical collecting tubule) was associated with the generation of a higher than normal electric potential gradient in the presence of CI but not of another poorly reabsorbable anion, such as SO₄^2-. These observations indicate that, in our patients, Henle’s loop function is normal, while the collecting tubule function is abnormal. We suggest that NaCl wasting and enhanced tubular secretion of H⁺ and K in our patients might result from an abnormally low conductance to CI in distal nephron site(s) where Na reabsorption is electrogenic, possibly the cortical collecting tubule. A larger than normal transtubular electric gradient would be generated by Na reabsorption, causing: (1) a direct stimulation of tubular secretion of K and H⁺ (leading to hypokalemia and alkalosis) and (2) inhibition of the reabsorption of Na (‘trapped’ into the tubular lumen by electric forces), with consequent extracellular volume contraction, hyperreninemia and hyperaldosteronism.