The diurnal antidiuretic hormone levels were studied in eleven enuretics and related to urine production and functional bladder capacity. A fluid deprivation test monitoring antidiuretic hormone levels was undertaken in four patients. The study suggests that the normal increase in nighttime antidiuretic hormone levels is absent in enuretics, who show a stable hormone level both day and night. Consequently the volume of night urine production approximates day urine production per hour. The functional bladder capacity was clearly exceeded at night in eight of eleven patients. The fluid deprivation test showed a normal response to fluid deprivation. In conclusion, the study adds further evidence that bladder capacity is a major factor in enuresis. Urine volumes that exceed bladder capacity at night may be caused by a lack of diurnal rhythmicity in antidiuretic hormone levels.