Patients suffering from nocturnal enuresis (starting from the age of 5 till adulthood) are all treated with the same dose of desmopressin, i.e. 120mcg once daily. In treatment resistant enuresis, this dose is doubled: those patients take 240mcg once daily. A pilot study performed at our department showed a correlation between weight and plasma concentration when a fixed dose of desmopressin oral lyophilisate formulation was given to the pediatric patient (older than 6 years). This study will investigate the pharmacokinetics (PK) and pharmacodynamics (PD) of desmopressin in young children, less than 8 years old. Additionally, the efficacy of desmopressin oral lyophilisate formulation in urinary concentration testing will be evaluated

Inclusion Criteria:

• Children with an uro- and/or nephropathy who need an urinary concentration test OR Children with monosymptomatic nocturnal enuresis (based on nocturnal polyuria) with treatment failure on desmopressin tablet
• Otherwise healthy children (on medical history and physical examination)
• Parents or legal guardian of the child signed the informed consent form
• Age: between 6 months and 8 years
• Minimum weight: 8 kg

Exclusion Criteria:

• Diabetes insipidus
• Renal failure (eGFR<60ml/min/1,73m²)
• Current urinary tract infection
• Syndrome of inappropriate antidiuretic hormone secretion
• Heart failure
• Clinical significant medical conditions (renal, hepatic, gastro-intestinal, pulmonary, cardiac, endocrinologic) that might interfere with the clinical endpoints
• Sensitivity to desmopressin or excipients of the oral lyophilisate formulation
• Use of antibiotics, diuretics or other drugs that can influence diuresis (tricyclic antidepressants, chlorpropamide, oxcarbazepine, selective serotonin reuptake inhibitors, chlorpromazine and carbamazepine).
• Use of drugs that influence intestinal motility (such as loperamide)
• Anomalies of the mouth that might interfere with the intake / absorption of the medication