The transition from day to night is associated with a pronounced decline in diuresis with reductions in the amount of excreted water, electrolytes, and other end products of our metabolism. Failure to do so leads to a large urine output at night, a condition known as nocturnal polyuria, encountered in a large proportion of children with nocturnal enuresis. The aim of this study was to clarify the mechanisms responsible for the nocturnal polyuria seen in enuretics with inadequate response to desmopressin (dDAVP). Forty-six enuretics (7-14 yr of age) and fifteen age-matched controls were admitted for a 24-h protocol with standardized fluid and sodium intake, comprising urine collections, blood sampling, and blood pressure monitoring. We included patients with severe enuresis (5 +/- 1 wet nights/wk) showing <50% reduction in wet nights on dDAVP. We characterized the patients on the basis of their nocturnal urine production. The children with nocturnal polyuria excreted larger amounts of sodium and urea at night than nonpolyurics and controls. Solute-free water reabsorption as well as urinary arginine vasopressin and aquaporin-2 excretion were normal in polyurics, and no differences were found in atrial natriuretic peptide, angiotensin II, aldosterone, and renin levels. Urinary prostaglandin E2 (PGE2) excretion was significantly higher in polyurics. The nocturnal polyuria in children with dDAVP-resistant nocturnal enuresis seems to be the result of augmented sodium and urea excretion. The high urinary PGE2 levels found in these children point toward a role for increased prostaglandin synthesis in the pathogenesis of enuresis-related polyuria.