In order to prevent hypovolemia fludrocortisone acetate treatment was started on admission in 39 consecutive patients with CT evidence of subarachnoid hemorrhage. In 28 patients an aneurysm was proven or probable, and in 21 of these the effects of fludrocortisone acetate on sodium balance and on plasma volume could be studied. In the first five days plasma volume decreased more than 10% in four patients, decreased less than 10% in five and increased in 12. The cumulative sodium balance measured over five days was negative in seven of the 21 patients. Plasma renin values were measured in 15 of the 21 patients and also in stored samples of 18 patients who were not treated with fludrocortisone acetate. Plasma renin values were less high in patients treated with fludrocortisone acetate, regardless of the presence of a negative sodium balance. In three of the 39 patients signs of pulmonary edema developed, and low serum potassium values were observed in four of the 21 patients. In comparison with previous studies, these findings suggest that fludrocortisone acetate is an effective method of decreasing the incidence of volume depletion and negative sodium balance.