Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are cardiac hormones, secreted by the atria and ventricles, respectively, in the normal adult heart. They participate in the regulation of blood pressure and body fluid homeostasis and modify growth and development of cardiovascular tissues and bone. Levels of ANP are higher in the fetal circulation than in adults, and fetal ventricles express higher levels of ANP and BNP than adult ventricles. The reappearance of ventricular ANP expression in adults is recognized as a marker of the induction of the embryonic gene program in ventricular hypertrophy. The natriuretic peptide system appears to be functional by midgestation, to respond to volume stimuli, and to regulate blood pressure and salt and water balance in the developing embryo. In addition, the natriuretic peptides may help regulate the blood supply to the fetus, acting as vasodilators in the placental vasculature. Peaks of ANP and BNP expression during gestation coincide with significant events in cardiac organogenesis, suggesting a role for ANP/BNP in the formation of the heart. In knockout mice lacking the natriuretic peptide receptor (NPR)-A gene (Npr1(-/-)), survival is reduced, with hearts enlarged at birth and possible cardiac developmental abnormalities. Surviving adult Npr1(-/-) mice have elevated blood pressure and marked cardiac hypertrophy and fibrosis, indicating that the ANP/BNP system is an important regulator of myocyte growth during development.