The plasma levels of atrial natriuretic peptide (ANP) have been determined by direct radioimmunoassay in 36 clinically-healthy subjects, 24 patients with compensated cirrhosis of the liver, and 20 patients with cirrhosis and ascites. When compared with controls, plasma levels of ANP in compensated cirrhosis do not demonstrate a significant difference (p greater than 0.05). Patients with decompensated cirrhosis of the liver show significantly (p less than 0.001) higher levels of ANP with respect to the controls and to compensated patients. In the control group, a significant (p less than 0.001) negative correlation between plasma levels of ANP and plasma renin activity and plasma aldosterone is demonstrated. The patients without ascites do not present any correlation between these variables (p greater than 0.05). The patients with ascites show a significant (p less than 0.01) positive correlation between ANP and plasma renin activity and plasma aldosterone levels. These results and other data suggest that ANP, although appearing to have plasma levels correlated with the disease stage, and with increases in both renin activity and plasma aldosterone levels, does not seem to have a very important role in sodium retention, or in genesis of ascites in cirrhosis of the liver. Activation of the renin-angiotensin-aldosterone system, activation of the sympathetic nervous system, and the circulatory modifications as well as other mechanisms seem to be more important in the formation of ascites in cirrhotic patients.