The etiology of hepatorenal syndrome is still incompletely understood, but the nonosmotic release of arginine vasopressin (AVP) seems to have an important role. Since AVP presents a well-defined daily periodicity in its production and secretion, the aim of the study was to investigate the circadian rhythm in its circulating plasma concentrations in patients with hepatorenal syndrome, compared with healthy controls. Venous blood samples were drawn during a 24 hour period at 2 hourly intervals from a peripheral vein in 10 healthy subjects and in 10 patients with hepatorenal syndrome for the determination of the circulating plasma levels of AVP by radioimmunoassay. Statistical analysis was carried out by the ‘cosinor’ method. The controls presented a significant (p < 0.002) circadian rhythm in the AVP circulating levels, whereas no rhythm (p > 0.05) was found in patients. The circadian rhythms were significantly (p < 0.005) different between the two groups, patients having higher mean daily concentration and lower circadian variation of AVP. These results confirm the existence of a well-defined circadian rhythm of AVP in healthy subjects, whereas the hepatorenal syndrome patients present a complete loss of the circadian rhythm. It could be hypothesized that primitive damage in circadian time-keeping regulates the circadian rhythm of vasopressinergic neurons, or has a secondary effect on the peripheral vasodilatation in the course of advanced liver disease. This great upset in the temporal and functional organization, together with that of the renin-angiotensin-aldosterone system, could play an important role in promoting and/or in the maintenance of the hydroelectrolyte imbalance that characterizes the hepatorenal syndrome.