Hyponatremia following the subarachnoid hemorrhage has been attributed to the syndrome of inappropriate secretion antidiuretic hormone or salt wasting syndrome. Recently discovered atrial natriuretic peptide(ANP) is known to contol sodium and extracellular fluid homeostasis by increasing renal excretion of sodium. To investigate whether the hyponatremia following the subarachnoid hemorrhage(SAH) is due to changes in plasma ANP, plasma ANP, serum sodium concentration and central venous pressure were measured in 10 patients(CSF ANP as well in some patients) with subarachnoid hemorrhage. The results obtained were as follows. 1) Plasma ANP concentration increased during the acute stage of SAH, being recovered to control levels after 8 days of SAH. 2) ANP concentration was significantly higher in plasma than in CSF. 3) No significant correlation was noted between the plasma ANP and CVP or serum sodium concentration. These results suggest that ANP is not involved in the development of hyponatremia during the acute stage of SAH.