A study of plasma arginine vasopressin in 17 patients with the syndrome of inappropriate antidiuretic hormone secretion (SIADH) associated with bronchogenic carcinoma, revealed that the arginine vasopressin levels were distinctly elevated in most. In 14 patients with bronchogenic carcinoma, but without overt SIADH, plasma levels of arginine vasopressin were significantly higher than in normal subjects (p less than 0.001). This, together with the finding of a lower than normal plasma osmolality in this group, suggests that inappropriate ADH excess might be much more common in patients with bronchogenic carcinoma than previously thought. The normal positive correlation between plasma osmolality and plasma arginine vasopressin was found to be reversed in SIADH. Seven of nine patients with overt SIADH, studied after fluid deprivation, showed an increase in plasma arginine vasopressin coincident with an increase in plasma osmolality (r = +0.8, p less than 0.01); in one patient, plasma arginine vasopressin returned to the original level following rehydration. The possibility that this might imply a degree of physiologic control to what is generally considered an autonomous secretion is discussed. It is, however, considered more likely that other factors, including changes in plasma volume and glomerular filtration, might explain the increase in plasma levels of arginine vasopressin.