The recent discovery of the atrial natriuretic peptide (ANP) system and its possible physiological role in man are discussed. ANPs are produced in the atria of various species, including man. In normal humans the biologically active alpha-ANP, a peptide consisting of 28 amino acid residues, is released into the circulation through cleavage from a larger prohormone which is stored in specific granules in atrial myocytes. The secretion of alpha-ANP from the heart is activated during atrial distension secondary to volume overload or increased atrial pressures. When injected intravenously, synthetic human alpha-ANP can produce a wide spectrum of hemodynamic, endocrine and renal effects. The ANP system may well play an important role in the homeostasis of (central) blood volume and blood pressure.