Brain natriuretic peptide (BNP) circulates in blood as a peptide hormone with natriuretic, vasodilatory and renin inhibitory properties. BNP is secreted predominantly by the left ventricular myocytes in response to volume expansion and pressure overload. BNP belongs to a family of structurally similar peptide hormones, which includes atrial natriuretic peptide (ANP), BNP, C-type natriuretic peptide (CNP) and urodilatin. These peptides are characterized by a common 17 amino acid ring structure with a disulfide bond between two cystein residues. This ring structure shows high homology between different natriuretic peptides (eleven of the 17 amino acid residues are homologous in the ring of each of the natriuretic peptides). BNP is a 32 amino acid peptide with disulfide bond between the cysteine residues Cys10 and Cys26. In earlier studies it has been demonstrated that BNP concentration in blood increases with the severity of congestive heart failure. Quantitative measurement of BNP in blood provides an objective indicator of congestive heart failure severity.