Nonspecific baroreflex loading maneuvers such as head-down till readily suppress stimulated arginine vasopressin levels in normal humans. To test the hypothesis that the increased arginine vasopressin levels in patients with congestive heart failure would not respond normally to baroreflex loading, 12 patients with congestive heart failure had arginine vasopressin levels and osmolality determined in the supine position and after 15 min of 30 ° head-down tilt. Arginine vasopressin was increased to 6.5 ± 2.0 pg/ml at control measurement and did not decrease. Eight patients underwent further study after osmotic stimulation with mannitol. Mannitol increased osmolality from 287 ± 9.2 to 294 ± 7.8 mOsm/kg (p < 0.001) and from 288 ±9 to 299 ± 8.2 mOsm/kg (p < 0.01) on two occasions. No significant suppression of arginine vasopressin was seen during head-down tilt after mannitol infusion compared with values in a time control period. These results are consistent with an abnormality in baroreflex suppression of arginine vasopressin secretion in chronic congestive heart failure and suggest that such a defect may contribute to long-term high levels of arginine vasopressin in this condition.