An oral water load of 20 ml (kg body wt)-1 was given to seventeen patients with the nephrotic syndrome and fifteen healthy control subjects. Diuresis (D), free water clearance (CH2O), plasma concentrations of arginine vasopressin (AVP), angiotensin II (AII) and aldosterone (Aldo) were determined before and 3 times during the first 4 h after loading. In the nephrotic syndrome D was significantly lower 1-2 h after loading than in the control subjects, predominantly due to a lower CH2O (2.61 and 7.01 ml min-1 (medians), P less than 0.01). Creatinine clearance and the maximum increase in CH2O were significantly correlated in patients with the nephrotic syndrome (rho = 0.721, n = 17, P less than 0.01) and the control subjects (rho = 0.596, n = 15, P less than 0.01). AVP was reduced in both groups during loading, but AVP was clearly elevated in the patients with the nephrotic syndrome when compared to the control subjects both before (3.0 and 1.9 pmol 1(-1), P less than 0.01) and during loading. There was a significantly negative correlation between CH2O and AVP in both groups. AII and Aldo were reduced during loading, but the levels were the same in the patients and in the control group, and AII and Aldo were not correlated to CH2O. It is concluded that patients with the nephrotic syndrome excrete an oral water load more slowly than healthy control subjects, and that this phenomenon partly is due to reduced glomerular filtration rate and partly to increased AVP.