The demonstration by De Bold, in 1981, that atrial extracts are able to produce a dramatic hypotensive and natriuretic effect led, within 5 years, to the identification and biosynthesis by genetic biotechnology of a new hormone: the atrial natriuretic polypeptidic factor or cardionatrin which is stored in the atrial granules previously described by Kisch. The secretion of this hormone is stimulated by distension of both right and left atria, and this explains why it plays a key role in cardiovascular homeostasis. Cardionatrin acts mainly as an antagonist of two pressive systems: the renin-angiotensin-aldosterone system and the adrenergic system. It acts by stimulating receptors which induce an increase of cyclic guanosine monophosphate and a decrease of cyclic adenosine monophosphate. Its natriuretic effect is explained not only by an increase in glomerular filtration rate but also by an increased secretion of papillar interstitial fluid into the collecting ducts.