[en] A sensitive and specific radioimmunoassay for α-human atrial natriuretic polypeptide (α-hANP) was developed to determine its plasma level. The plasma α-hANP level in normal subjects fell significantly during water deprivation and increased significantly on infusion of hypertonic saline. The mean plasma levels of α-hANP were higher than normal in patients with essential hypertension, liver cirrhosis, congestive heart failure and chronic renal failure. The authors results indicate that this radioimmunoassay is suitable for determining the α-hANP concentration in human plasma and can assess changes in pathological and physiological states. (Auth.)