OBJECTIVE The causes of volume depletion and hyponatremia after subarachnoid hemorrhage are not fully understood but may be in part due to natriuresis or "cerebral salt wasting." Because previous studies using infrequent hormone sampling have given inconsistent results, we determined if elevations in atrial natriuretic factor concentrations preceded negative sodium and fluid balances. METHODS We measured diurnal atrial natriuretic factor and vasopressin concentrations and sodium balance for 5 days in 14 consecutive patients after aneurysmal subarachnoid hemorrhage. RESULTS Plasma concentrations of atrial natriuretic factor on admission were elevated in subarachnoid hemorrhage patients (mean +/- SD 106 +/- 59 pg/ml) compared with acutely ill controls (39 +/- 30 pg/ml). In eight patients, high peak concentrations of atrial natriuretic factor, greater than 300 pg/ml or a twofold increase above baseline, were followed by natriuresis and a negative sodium balance. Three patients, two of whom became hyponatremic, developed cerebral infarcts after natriuresis. Vasopressin concentrations were slightly elevated just after hemorrhage but subsequently declined to normal values. CONCLUSIONS A markedly increased atrial natriuretic factor concentration precedes natriuresis in some patients and, with other abnormalities of water handling possibly including a relatively diminished vasopressin concentration, may cause volume depletion. Patients with natriuresis appear to be at increased risk for delayed cerebral infarction after subarachnoid hemorrhage.