The investigators want to test the hypothesis that patients with chronic renal disease have a poorer ability to preserve water after being thirsty and a poorer ability to excrete water after a load of fluid. They presume that these abilities become poorer when renal insufficiency progresses. The investigators further hypothesize that patients with hypertension also have a decreased ability to concentrate and dilute urine.

Inclusion Criteria:

• Patients with chronic renal insufficiency, aged between 18 and 65, both men and women
• Creatinine clearance between 30-59 ml/min
• Patients with hypertension, aged between 18 and 65, both men and women; hypertension is defined as a mean arterial blood pressure in the daytime that is more than 140/90 mm Hg.
• Healthy volunteers, aged between 18 and 65, both men and women

Exclusion Criteria:

• Clinical signs or history of disease in the heart, lungs, liver, brain, and endocrine organs
• Cancer
• Alcohol abuse
• Medicine abuse
• Pregnancy

Healthy participants were recruited by advertisements in public and private institutions Patients with primary hypertension or chronic renal failure were recruited from the Out-patients' Clinic, Department of Medicine, Section of Nephrology, Holstebro Hospital