Atrial natriuretic peptide (ANP) is a 28 amino acid peptide that is synthesized, stored, and released by atrial myocytes in response to atrial distension, angiotensin II stimulation, endothelin, and sympathetic stimulation. The presence of atrial natriuretic peptide (ANP) and the nature of Atrial Natriuretic Peptide's binding sites were studied in fresh-water (FW)- and seawater (SW)-adapted eels using a heterologous analogue, that of the rat (rANP).