The ventricular myocardium simultaneously secretes two natriuretic peptides useful in the evaluation of heart failure: BNP, hormonally active, and NT-proBNP, the N-terminal end, non-hormonally active, but ultimately their concentrations differ and their clearance patterns are poorly defined.We measured NT-proBNP and BNP in patients with and without heart failure and compared their concentrations using regression analysis.The relationship between NT-proBNP with BNP is nonlinear. Between 45 and 70 pmol of BNP/L (class II heart failure) the slope is much higher than in other ranges and the NT-proBNP/BNP ratio reaches its maximum in patients with class II NYHA heart failure.The difference in concentration for NT-proBNP and BNP can be related to the diffusion across the renal basement membrane. Their ratio is nonlinear because BNP is cleared faster than in patients with class II heart failure than other classes or normal, suggesting a change in a non-renal mode of clearance.