AIMS: Plasma B-type natriuretic peptide (BNP) and proBNP are promising markers for treatment of heart failure (HF), but the intra-individual biological variation is high. We investigated whether changes in sodium intake and posture contribute to this variation. METHODS AND RESULTS: A total of 12 healthy individuals and 12 patients with medically treated compensated HF were examined after 1 week of low (70 mmol [1.61 g] per day) and 1 week of high (250 mmol [5.75 g] per day) sodium intake. Plasma volume and plasma concentrations of BNP and proBNP were determined after 1 h in seated and 1 h in supine position. In healthy individuals, the plasma BNP concentration increased significantly on high sodium intake with a ratio (high sodium/low sodium) of 2.00 (1.32-3.03, P = 0.004). The corresponding values for HF patients were 1.69 (1.25-2.29, P = 0.003). The plasma BNP concentration changed modestly by a posture change, with a plasma BNP ratio (supine/seated) of 1.15 (1.07-1.14, P = 0.001) and 1.06 (0.99-1.24, P = 0.088) in healthy subjects and HF patients, respectively. Plasma proBNP concentrations were neither significantly affected by posture nor by sodium intake. CONCLUSION: Sodium intake has a considerable effect on plasma BNP and therefore contributes to the intra-individual variability. We suggest dietary sodium intake to be standardized at least 3 days prior to blood sampling for the determination of plasma BNP.