Because both atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) competitively bind to natriuretic peptide receptors but not N-terminal proBNP (NT-proBNP), the diagnostic value of BNP as a marker of the severity of heart failure in comparison with NT-proBNP during exogenous ANP (carperitide) infusion remains unclear. Forty-two patients with CHF (NYHA class III or IV) treated with the infusion of carperitide were included in the present study. We measured plasma levels of BNP and NT-proBNP at baseline and after the improvement of symptoms. We also measured these parameters before and 1 hour after stopping the infusion of carperitide. After stopping the infusion of carperitide, the plasma BNP level was significantly decreased by about 20% (394 +/- 53.8 versus 312.8 +/- 46 pg/mL, P < 0.0001) but plasma NT-proBNP did not change (1674.5 +/- 282.1 versus 1777.5 +/- 300.3 pg/mL, P = 0.259). The molar ratio of plasma BNP/NT-proBNP was significantly higher during carperitide infusion (0.74 +/- 0.08) than those at baseline (0.63 +/- 0.06) and after stopping carperitide (0.59 +/- 0.07). During carperitide infusion, plasma NT-proBNP may be a more reliable marker of endogenous cardiac natriuretic peptides than plasmaBNP, which may be increased by carperitide infusion.