The syndrome of inappropriate secretion of antidi uretic hormone (SIADH) is characterized by hypona tremia primarily due to water retention and with natriuresis closely parallel to sodium intake [1]. The mecha nisms that govern the natriuresis in that condition have not been clearly delineated. To determine whether hu moral factors have a role in the natriuresis we measured plasma atrial natriuretic factor in a patient with hypo natremia due to SIADH that was associated with head trauma. The patient, a 51-year-old man, was investigated in the basal state under water restriction and after water loading. In the basal state the serum electrolyte levels (expressed in millimoles per liter) were as follows: so dium 123, potassium 4.9. and chloride 84. The plasma urea level was 2.7 mmol/1 and the creatinine clearance was 123 ml/min. Plasma and urine osmolality measured simultaneously were 250 and 754 mosm/kg, respective ly. The urinary sodium concentration was 130 mmol/1 and the fractional excretion of sodium was 0.51 %. Thyroid, pituitary and adrenal functions were nor mal. Plasma renin activity (0.6 ng/ml/h) was below the normal range. Catecholamines and plasma aldosterone level (33 pg/ml) were in the normal range. The plasma levels of AVP and of ANF were elevated at 5.4 and 60.5 pg/ml, respectively. Under water restriction the se rum sodium level and plasma osmolality increased to 139 and 280, respectively. Weight decreased from 55 to 52 kg. Plasma ANF decreased to 45.5 pg/ml. Plasma aldosterone level, PRA and plasma cathecholamine lev els remained unchanged. The response to an acute oral water load (20 ml/kg body weight in 30 min) was clearly abnormal: only 20% of the oral load was excreted in 4 h (normal excretion, 80%). The minimal urinary osmolality was 878 mosm/kg (normal value. < 100 mosm/kg); thus, free water clear ance remained negative throughout the test. The plasma osmolality decreased to 267 mosm/kg, a value below the