Atrial natriuretic factor (ANF) is a recently discovered peptide present in secretory granules specifically found in atrial muscle cells. Multiple structurally related peptides have been isolated from atrial tissues, all of which are derived from a common 152-amino-acid precursor. ANF induces profound natriuresis and diuresis in experimental animals and also causes relaxation of precontracted vascular smooth muscle. ANF has striking renal hemodynamic actions (most consistently an increased glomerular filtration rate), which probably explain its natriuretic effects. ANF also can inhibit renin secretion in vivo and causes direct inhibition of basal and stimulated aldosterone production. It lowers arterial blood pressure, probably reflecting in part its vasorelaxant actions, and this effect is particularly marked in renin-dependent (and possibly other vasoconstricted) models of hypertension. Although the exact structure and regulation of the presumed circulating form(s) of ANF remain to be clarified, available information suggests that it may be a new, previously unrecognized factor in the regulation of fluid volume and renal and cardiovascular function.