Two independent series of biomedical investigations have led to the discovery that the atria are a peptide-secreting endocrine gland. The first is mainly morphological and starts with the finding that mammalian atrial but not ventricular cardiocytes contain "dense bodies". These "dense bodies" later called "specific granules" were found to be different from lysosomes, to be made up of proteins and to incorporate both 3H-leucine and 3-H-fucose in a pattern typical of peptide-secreting endocrine cells. The finding that rat atrial granulation varied with the sodium and water balance led to the crucial observation that atrial extracts have natriuretic and diuretic effects. In less than 4 years, this new natriuretic hormone has been purified, sequenced and synthetized, and its cDNA and gene have been cloned. The ANF gene has been assigned to the distal short arm of chromosome 1 in band 1P36 while the mouse gene is localized in chromosome 4. The native and synthetic hormones exert identical wide ranging effects (possibly through particulate guanylate cyclase stimulation and adenylate cyclase inhibition) on the kidney, blood vessels, adrenal cortex and pituitary. Physiopathologic implications of the hormone in experimental hypertension, congestive heart failure and expansion of blood volume are already beginning to emerge. On the other hand, the search for natriuretic hormones or factors by studies of negative pressure breathing, atrial distention experiments, head-out water immersion, expansion of blood volume, Na+/K-ATPase inhibition and parabiosis experiments in Dahl rats has provided a general framework within which to interpret this new cardiac function.