The mammalian atrial hormone atrial natriuretic factor (ANF) has been shown to have potent natriuretic and diuretic actions as well as vasodilator effects when released into the circulation. To investigate how the levels of the circulating form of this peptide c$ cular fluid volume, we measured immunoreactive ANF in the plasm$ load, acute furosemide treatment, and chronic water restriction. Cii$ ANF increased significantly (p< 0.001) 1 minute after acute saline 1$ within 5 minutes. Volume contraction induced by furosemide treat$ significantly reduced the circulating immunoreactive ANF. These$ expansion causes an immediate release of immunoreactive ANF intt$ volume contraction results in a decline of circulating levels of imm$ tained during chronic volume contraction. These results suggest t$ intravascular fluid volume and respond by changing the levels of A$ and thereby participate in the regulation of body fluids and, perl$.