A patient with the syndrome of chronic hypernatremia (serum Na+: mean = 154, range 139-184 mEq/l, n = 30) and hypodipsia due to a hypothalamic injury was studied to evaluate osmolar and baroreceptor control of arginine vasopressin (AVP) secretion. Resting plasma AVP levels measured by radioimmunoassay were inappropriately low for the degree of plasma hyperosmolality: range = less than 0.5-2.1 pg/ml, n = 10, with corresponding levels of plasma osmolality (P osM) greater than 300 m osmol/kg, suggesting either direct damage to the AVP synthesis and storage area or impaired afferent osmoreceptor function. Direct pituitary damage seemed unlikely, since anterior pituitary function was normal by standard testing. The existence of adequate neurohypophyseal stores of AVP was demonstrated by baroreceptor stimulation with the hypotensive agent trimethaphan (Arfonad): plasma AVP rising to 50.0 pg/ml during transient hypotension (BP = 70/0). Osmoreceptor function was evaluated during acute water loading followed by hypertonic saline infusion. During hypertonic saline infusion plasma AVP levels correlated with P osM (R = .87, P less than .01, n = 8), suggesting some residual osmotic regulation of AVP release. The osmotic threshold for AVP release (the x-axis intercept of the plasma AVP-P osM regression line) was not higher than normal. However, the AVP levels throughout this study remained markedly subnormal for the degree of plasma hyperosmolality (maximum plasma AVP = 1.9 PG/ML when P os M = 327 M OSMOL/KG). Since a substantial amount of AVP was released with baroreceptor stimulation, the inadequate rise in plasma AVP level with hyperosmolality indicates that afferent input from the osmoreceptor/thirst area of the hypothalamus is selectively impaired in this patient. These findings directly demonstrate a dissociation of osmoreceptor function from the AVP secretory apparatus in man.