Mammalian distal tubules adapt structurally and functionally when NaCl concentration in tubule fluid is altered chronically. These experiments were designed to test the hypothesis that chronic administration of hydrochlorothiazide (HCTZ), a drug that blocks Na and Cl uptake across apical membranes of rat distal tubule cells, would reduce intrinsic transport capacity of distal tubules and reduce the number of thiazide-sensitive transporters. Osmotic pumps were implanted into rats to deliver 3.75 mg/day HCTZ or vehicle for 10-14 days. All animals were offered a solution containing 0.8% NaCl and 0.1% KCl as drinking fluid. Free-flow micropuncture after 10–14 days indicated that Na and Cl delivery to distal tubule was not significantly different in HCTZ- and vehicle-treated animals. Microperfusion in vivo with an artificial interstitial solution, with no thiazide, indicated that 10-14 days of HCTZ infusion did reduce Na transport capacity of distal tubules from 390 +/- 32 to 203 +/- 24 pmol/min (P less than 0.01). In contrast, the number of thiazide-sensitive NaCl transporters, determined as high-affinity receptors for [3H]metolazone in renal cortical membranes, was higher in HCTZ group than in controls (2.2 +/- 0.4 vs. 1.0 +/- 0.1 pmol/mg protein, P less than 0.01). These data support the hypothesis that chronic blockade of NaCl entry across apical membranes of distal tubule cells reduces NaCl transport capacity, an effect that occurs despite an increase in the number of thiazide receptors. They indicate that thiazide receptor binding studies should be interpreted in combination with direct functional measurements.