Body fluid homeostasis is maintained by the kidney. Such an accurate control in achieved via the secretion of antidiuretic hormone (ADH), the secretion of which is regulated by hypothalamic osmoreceptors. Both urine flow rate and the excretion of most electrolytes have a diurnal rhythm; they increase during daytime and decrease during nighttime. Such a rhythm seems to be absent in some subjects who suffer from bedwetting because of relative polyuria. In these cases, the polyuria is associated with a decreased nocturnal secretion of ADH and the subsequent excretion of dilated urine. A deficit in the nocturnal secretion of ADH thus appears to explain the response to desmopressin of children with a polyuric form of enuresis.