Angiotensin II(AII), aldosterone (Aldo) and arginine vasopressin (AVP) in plasma were determined during basal conditions in seventeen patients with congestive heart failure and in seventeen control subjects. The same parameters were measured before and 1, 2 and 3 h after an oral water load of 20 ml (kg body weight)^‐1 together with urine volume (V) and free water clearance (C_H2_O) in seven patients with congestive heart failure and in seven control subjects. AII, Aldo and AVP were significantly higher in heart failure than in control subjects (AII: 81 and 12 pmol l^‐1 (medians), P < 0·01; Aldo: 411 and 103 pmol l^‐1, P < 0·01; AVP: 5·3 and 2·0 pmol l^‐1, P < 0·01). AVP was positively correlated to Aldo in both heart failure (ρ= 0·593, n= 17, P < 0·02) and control subjects (ρ= 0·511, n= 17, P < 0·05), but in neither of the groups to AII. V and C_H2O were significantly lower in heart failure when compared to control subjects (maximum increase in C_H2O 3·55 and 5·86 ml min^‐1, P < 0·02), but did not correlate directly with either A II, Aldo or AVP. Creatinine clearance was reduced in heart failure. It is concluded that the activity of both the renin‐angiotensin‐aldosterone system and the osmoregulatory system is enhanced in congestive heart failure, presumably as a compensatory phenomenon in order to maintain arterial blood pressure. It is suggested that the decrease in free water clearance may be attributed to both an elevated level of vasopressin and a reduced glomerular filtration rate.