The authors describe a case of subarachnoid hemorrhage with hyponatremia accompanied by elevation of plasma atrial natriuretic peptide (ANP). The early phase of hyponatremia was classified as the syndrome of inappropriate secretion of antidiuretic hormone (ADH) due to subarachnoid hemorrhage. However, in the later phase, hyponatremia and natriuresis were accompanied by suppression of ADH while plasma ANP remained elevated. The patient was effectively treated with demeclocycline and hypertonic saline. The significance of ANP in the pathophysiology of increased natriuresis is discussed.