The atria secrete atrial peptides (atriopeptins) that are capable of producing dramatic alterations in a number of body processes. Secretion of atriopeptin appears to be regulated primarily by the prevailing pressure within the atria. In pharmacological doses, atriopeptin rapidly elicits a natriuresis when administered to experimental animals or humans. In contrast, infusion rates that increase plasma atriopeptin only by about three- to fivefold tend to produce a slowly developing and modest diuresis. Evidence examined in this review suggests that the atrial peptides are not potent natriuretic substances under normal physiological conditions, although it is likely that they exert a modulating influence on sodium excretion. The atrial peptides are vasoactive and induce a number of cardiovascular changes including decreases in arterial blood pressure, cardiac filling pressure, and cardiac output and a translocation of fluid from plasma to the interstitial fluid space. They also interact with other hormones, particularly the renin-angiotensin-aldosterone system. Finally, atriopeptin is distributed throughout many regions of the brain where it may serve as a neuromodulator or neurotransmitter. Atriopeptin circulating in the bloodstream also may influence cerebral mechanisms by acting on receptors in the circumventricular organs. A more complete understanding of the diverse effects of this cardiac endocrine system is expected to provide further insight into a spectrum of physiological and pathophysiological processes.