Hyponatremia is common following subarachnoid hemorrhage and has alternatively been attributed to either the inappropriate secretion of antidiuretic hormone or natriuresis causing intravascular volume contraction. We prospectively studied body sodium and intravascular volume regulation in 19 patients, beginning within 3 days after acute aneurysmal subarachnoid hemorrhage occurred, in order to determine the impact of hypervolemic therapy on both hyponatremia and volume contraction and to ascertain whether humoral factors account for hyponatremia. Serial measurements of plasma arginine vasopressin, atrial natriuretic factor, renin activity, aldosterone, and catecholamines were correlated with body sodium and fluid balance, change in blood volume, serum sodium concentration, and osmolality. Six patients (32%) developed hyponatremia, but only 2 had a negative sodium balance. In most patients, levels of atrial natriuretic factor were elevated, while plasma renin activity and aldosterone concentrations were generally suppressed. Plasma arginine vasopressin levels were not suppressed during hypo-osmolality and did not correlate with serum osmolality in hyponatremic patients. Only 1 patient had a decrease in blood volume, which was associated with marked rises in aldosterone and plasma renin activity, but normal serum sodium and plasma atrial natriuretic factor levels. We conclude that following subarachnoid hemorrhage: (1) Hypervolemic therapy prevents volume contraction but not hyponatremia, (2) humoral factors may favor both sodium loss and water retention, and (3) arginine vasopressin regulation is disturbed and may contribute to hyponatremia.