Hyponatremia is a very common electrolyte abnormality in the neurosciences intensive care unit and in particular after subarachnoid hemorrhage. It is often associated with intravascular volume contraction. Evaluation and treatment are discussed in this chapter. Treatment of the polyuric, hyponatremic patient should include replacement of sodium and fluid volume. The goals of fluid management in cases of aneurysmal subarachnoid hemorrhage are maintenance of normonatremia and euvolemia. How to calculate rate of infusion of fluids is also highlighted in the chapter. It is important to replace volume effectively, but being careful not to induce fluid overload. Fludrocortisone is useful to ameliorate urinary sodium loss and hyponatremia.