In 8 adult patients with monosymptomatic nocturnal enuresis (age 18-44 years) we twice investigated the circadian rhythm of plasma vasopressin, plasma atrial natriuretic peptide, serum osmolality, serum electrolytes as well as urinary excretion in relation to urodynamic variables. Seven of the patients showed a lack of diurnal rhythmicity in plasma vasopressin at both diurnal studies with no night-time increase. The nocturnal urinary volumes exceeded the bladder capacities with an average of 155%. This was mainly caused by a large excretion of poorly concentrated urine during the first hours of sleep. Although a slight nocturnal peak in plasma atrial natriuretic peptide was found no clear pathophysiological role could be determined for this peptide. It is concluded that the previously established vasopressin production abnormality in juvenile enuresis seems to persist in adult enuretics.