Recent research has led to the discovery and characterization of a hormone secreted by the atria that has powerful vasodepressor and natriuretic properties. A series of atrial peptides varying only in length and exhibiting these activities have been isolated, so that it is not yet clear which of these is the active hormone. The factors that determine its secretion remain to be characterized although preliminary evidence suggests that sodium-volume loading and/or intraatrial distension may be centrally involved. Atrial hormone acts in four different ways to oppose or counter the actions of the renin angiotensin aldosterone system. Thus, it produces vasorelaxation, which is especially pronounced in angiotensin-preconstricted blood vessels; it blocks angiotensin-induced aldosterone secretion by the adrenal cortex; it inhibits renin secretion by the kidneys; and its natriuretic action opposes the sodium-retaining action of aldosterone. Accordingly, the atrial-natriuretic and vasorelaxant hormone may play a role complementary to the renin angiotensin-aldosterone system in the long-term regulation of sodium balance and arterial pressure. In this construction the renin system acts primarily to defend sodium balance and blood pressure, with the atrial hormone playing an increasingly active counterpart in situations involving sodium-volume surfeit and/or high blood pressure. The physiologic properties of the new atrial hormone already suggest a major role for it in sodium-volume, blood pressure homeostasis, and for understanding and treating hypertensive-cardiovascular diseases.