In the anesthetized, normally hydrated dog, the i. v. infusion of norepinephrine (NE; 0.5 µg/kg/min) resulted in an increased blood pressure and a marked reduction in the plasma vasopressin (ADH) concentration (2.6 ± 0.2 to 1.1 ± 0.2 µU/ml). Urine flow and osmolar clearance increased, and urine osmolality fell. However, the negative free water clearance (TCH2O) increased, despite the reduction in plasma ADH levels. Thus, the NE-induced diuresis appeared to be due largely to the increased solute excretion, but the reduction in plasma ADH levels may also have been a factor. These data show that change in free water clearance is not a satisfactory index of change in the plasma ADH concentration when there are acute changes in renal hemodynamics and solute excretion. The norepinephrine-induced reduction in ADH secretion appeared to be due largely to increased activity of the arterial baroreceptors, but a central action cannot be ruled