Abstract It is well known that posture affects natriuresis in cirrhosis and heart failure. This study evaluates the role of posture on spontaneous urinary salt excretion (UNa V) and diuretic-induced natriuresis in nephrotic patients with mild renal impairment. UNa V and plasma concentrations of the main hormones involved in sodium regulation were evaluated at baseline (Baseline) and after furosemide administration (20 mg intravenously at 8:00 am [Diuretic]) in seven nephrotic patients with mild renal impairment (creatinine clearance, 68.5 ± 7.6 mL/min) in either the supine or upright position for 6 hours (from 8:00 am to 2:00 pm ). At baseline, UNa V was greater in the supine than upright position (sodium, 51.8 ± 6.2 versus 38.3 ± 6.1 mEq/d; P < 0.01). Similarly, furosemide was more effective in increasing UNa V in the supine (sodium, 51.8 ± 6.2 to 87.4 ± 9.1 mEq/d; P < 0.005) than upright position (sodium, 38.3 ± 6.1 to 59.0 ± 6.8 mEq/d; P = not significant). Consequently, body weight decreased in the supine but not the upright position (–0.73 ± 0.15 versus –0.17 ± 0.22 kg; P < 0.05). Peripheral renin activity (PRA) and plasma aldosterone (Aldo) concentrations were greater in the upright than supine position at both Baseline and Diuretic. A similar pattern was observed for hematocrit, used as an index of plasma volume. In addition, a positive correlation was detected between hematocrit and PRA (r = 0.89; P < 0.001) in the upright position. Postural changes did not influence plasma concentrations of atrial natriuretic peptide. These data indicate that in nephrotic patients with mild impairment of glomerular filtration rate, the upright position causes a reduction in plasma volume; this hypovolemia activates the renin-Aldo system responsible for sodium retention in unstimulated conditions and a blunted natriuretic response to furosemide.