Renal function and plasma antidiuretic hormone (ADH) levels were studied basally and after oral water load in four groups of subjects: 15 healthy controls (group I), 15 cirrhotics without ascites (group II), 15 cirrhotics with ascites (group III), and 10 decompensated cirrhotics with hyponatremia (group IV). Renal function and ADH levels were normal in group II. In groups III and IV water diuresis and fractional proximal sodium excretion were significantly decreased, whereas fractional distal sodium resorption and fractional excretion of potassium did not differ from those of controls. Basal ADH was significantly increased only in patients of group IV. In these patients ADH remained abnormally high after water loading. ADH did not correlate with water diuresis, plasma osmolality, mean arterial pressure, and plasma renin activity. We conclude that impaired water excretion in decompensated cirrhotics without hyponatremia cannot be ascribed to high serum levels of ADH. On the contrary, it seems to be related mainly to a reduced delivery of filtrate to the diluting segment of the nephron. In cirrhotic patients with hyponatremia high levels of ADH may play an additional role.