Hypernatremia is the most common cause of hypertonicity in small animal medicine. Despite this fact, severe hypernatremia is an uncommon clinical entity in dogs and cats. The causes of hypernatremia are excessive water loss and increased sodium intake. Clinical signs are most often related to CNS dysfunction. Severe hypernatremia should be considered a life-threatening situation and treated as such. Initial fluid therapy should be given with care according to the rate of onset of hypernatremia, as deterioration of the animal's condition is a common sequela. The determination of the cause of hypernatremia and the treatment can be both challenging and rewarding. The other notable hypertonic syndrome in small animal medicine is hyperosmolar nonketotic diabetes mellitus. Judicious management of this disease with fluid therapy and insulin is the standard therapy. An understanding of the pathophysiology is essential to their diagnosis and appropriate medical management.