Diurnal changes of extracellular body water in enuretic (n = 8) and healthy children (n = 8) and plasma antidiuretic hormone level were examined in enuretic patients, using bioelectrical impedance analysis and radioimmunoassay. In enuretic children day/night values of extracellular space were 25.81% (10.90 l) vs 25.00% (10.52 l), with a night reduction of 3.13% (daytime 100%) (0.38 l). In controls the same parameters were 24.92% (11.88 l) vs 24.82% (11.80 l), the difference is 0.44% (0.08 l). In patients plasma antidiuretic hormone values were 2.96 pM/ml during the day and 2.70 pM/ml in the night. Results show, that in enuretic children there is a nocturnal reduction in extracellular water (p < 0.01), and the physiological nocturnal rise in antidiuretic hormone secretion in absent.