Ongoing investigations by our laboratory group and others have established that the heart is an endocrine organ as well as a pump. The heart synthesizes and secretes two peptide hormones - atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP) - that are endogenous ligands for a particulate guanylyl cyclase receptor (NPR-A). Following receptor binding and generation of its second messenger cGMP, the natriuretic peptides (NPs) mediate biological actions which include natriuresis, inhibition of the renin-angiotensin system and vasodilatation with local autocrine and paracrine actions in the heart to include inhibition of fibrosis and enhancement of diastolic function.