Peculiarities of the excretion of ions (Na, K, Ca, Mg) and water were studied in healthy children and children with the nocturnal enuresis, aged 6-15 years. A greater diuresis in the enuretic children is due to an increased excretion of the osmotically active substances including Na and Mg; excretion of K and Ca does nor differ from the control. A new formula is proposed for the quantitative evaluation of the role of different substances in the osmolar clearance. A high correlation is found between the sodium and magnesium excretion and the osmotic free water reabsorption. A single intranasal administration of 1-deamino-8-D-arginine-vasopressin (DDAVP) to the children before their going to bed returned to the norm the sodium and magnesium excretion in the enuretic children. It is suggested that the defect peculiar to this particular pathology is associated with a decrease in the ion reabsorption in the thick ascending Henle loop. The normal level of the ion transport is restored after stimulation of V2-receptors by DDAVP. An explanation is suggested of the mechanism of the increase of diuresis with a simultaneous rise in the osmotically free water reabsorption in children with enuresis.