Patients with hyponatremia of central origin were treated with a mineralocorticoid, and the pathogenetic mechanism of the hyponatremia was assessed based on the therapeutic effect obtained. The subjects were 14 patients (6 with subarachnoid hemorrhage, 3 with hypertensive intracerebral hemorrhage, 2 with cerebral infarct and 3 with head injury) who developed hyponatremia, as a complication during their hospital stay for their intracranial lesions from April to December 1992. Patients with serum Na levels below 135 mEq/l for more than 2 days with no other discernible etiology were defined as having hyponatremia of central origin. The mineralocorticoid used was fludrocortisone acetate, and as a rule administration was started the day after the onset of hyponatremia. When improvements occurred within 3 days, in 3 to 7 days, or 8 days or more efficacy was rated "excellent", "good" or "poor", respectively. The mean interval until the onset of hyponatremia was 10.4 days, its mean duration was 5.7 days, and the mean minimum serum sodium level was 129.5 mEq/l. The dose of fludrocortisone administration was 0.1 mg/day except for one patient who was treated with 0.3 mg/day. The mean period of administration was 3.7 days (range: 3 to 6 days), and the route was via a stomach tube in 5 cases and oral in 9 cases. The therapeutic effect was excellent in 9 cases, good in 3 cases and poor in 2 cases, the efficacy rate being 86%. None of the patients manifested side effects. Plasma atrial natriuretic peptide levels were above 100 pg/ml in 2 patients and 50-100 pg/ml in 8 patients and neither of the former 2 patients exhibited "excellent" efficacy.(ABSTRACT TRUNCATED AT 250 WORDS)