Abstract: Objective: To study the clinical characteristics of primary hypernatremia.Methods: Clinical data of 2 patients diagnosed as primary hypernatremia were retrospectively analyzed.Results: The two patients who had injury of hypothalamus-pituitary area were characterized by persistent hypernatremia and hyperosmolality without thirsty sensation.After dehydration by performing water deprivation,urinary osmolality increased with and finally exceeded plasma osmolality increasing,but far from the normal range.The plasma osmolality of both inhibiting and stimulating of ADH release was much higher than the normal revealed by water loading test.Oral dihydrochlorothiazide,chlorpropamide and vasopressin administration could partially improve hypodypsia,hypernatremia and hyperosmolality.Conclusion: The proposed pathogenesis of primary hypernatremia is up-resetting of osmostat,which may be explained by combination of central partial diabetes insipidus and hypodypsia.Dihydrochlorothiazide,chlorpropamide and vasopressin could partially improve primary hypernatremia.