1. The antidiuretic potencies of blood, serum and plasma obtained from various parts of the cardiovascular system of rats were compared. 2. External jugular blood was found to have higher activity than blood from the other sites. After hypophysectomy the antidiuretic potency of external jugular blood was much diminished. 3. The antidiuretic activity of hepatic and renal venous blood was lower than that of arterial blood, suggesting that antidiuretic hormone is removed from blood during passage through the splanchnic vascular area or the kidneys, or both. 4. The antidiuretic potency of blood was shown to increase with the volume of blood withdrawn. 5. Ether anaesthesia increased the antidiuretic activity of external jugular blood. 6. The antidiuretic activity of carotid serum when given subcutaneously was higher than that of carotid plasma. This difference was not seen after intravenous injections. 7. In sheep and in mice, plasma of blood from the external jugular veins had more antidiuretic effect than plasma from other sites.