Hyponatremia is a common feature after subarachnoid hemorrhage. Hyponatremia is complex in its origin because different neuroendocrine disturbances are involved: elements of inappropriate secretion of ADH, cerebral salt wasting, and blunted response of the reninangiotensin-aldosterone system may occur simultaneously. Hyponatremia is accompanied by hypovolemia which implies a major risk for vasospasm and cerebral infarction. Hyponatremia itself might cause cerebral edema and intracranial hypertension. Fluid restriction is therefore contraindicated in hyponatremia following subarachnoid hemorrhage because of the negative impact on intravascular volume. On the contrary, replacement of both volume and sodium should be vigorously accomplished.