1. Extracts were prepared from bovine hypothalamus and cerebral cortex by gel filtration on Sephadex G-25 and G-50.
2. The vasopressin in the hypothalamic extracts was inactivated with thioglycollate and the effectiveness of inactivation was tested in the alcohol-anaesthetized rat.
3. The inactivated hypothalamic extracts caused a significant rise, and the cortical extracts an insignificant fall, in the urinary sodium excretion of the conscious rat.
4. Incubation of tubule fragments in hypothalamic extracts caused a significant rise in intracellular sodium concentration of the tubules when compared with incubation in Ringer, whereas incubation in cortical extracts caused a rise which was not significant. Nevertheless the rise in intracellular sodium concentration produced by incubating the tubules in hypothalamic extracts was not significantly different from the rise produced by incubation in cortical extracts.