Patients with chronic heart failure (CHF) have decreased ability to excrete water and have increased urinary excretion of aquaporin 2 (U-AQP2). The natriuretic and diuretic effects of furosemide are antagonized by an increased reabsorption of sodium and water in the collecting ducts. It is unknown if aquaporin-2 (AQP2) renal water channels are involved in this compensatory reabsorption. We tested the hypothesis that U-AQP2 increases after a single intravenous dose of furosemide in CHF patients. In a randomized, single blind, placebo controlled, cross over study, we measured the effect of furosemide, 80 mg, on U-AQP2, urine volume, free water clearance (CH2O), and fractional excretion of sodium (FENa) in twelve CHF patients. Plasma concentrations of vasopressin (AVP), renin (PRC), angiotensin II (Ang II), aldosterone (Aldo), atrial (ANP) and brain natriuretic peptides (BNP) were measured during the study. U-AQP2 and hormones were determined by radioimmunoassays. Furosemide increased U-AQP2 (140%), urine volume (280%), CH2O (95%), and FENa with a factor of 15 (P<0.008 for all). Furosemide increased AVP (51%), PRC, Ang II (86%), and Aldo (59%), (P<0.021 for all). ANP and BNP did not change. After placebo Aldo decreased 18% (P<0.023), and the other variables were unchanged after placebo.In CHF patients furosemide increased U-AQP2 indicating increased water reabsorption in the distal part of the nephron. This is most likely a compensatory phenomenon in addition to increased activity of the renin-angiotensin-aldosterone system, to prevent excess loss of sodium and water.