1. In anaesthetized dogs urinary bladder distension caused variable changes of mean blood pressure; these were unaffected by vagotomy.
2. Catecholamines were detected in the circulation during bladder distension in fourteen of seventeen dogs. The catecholamine concentration during bladder distension was greater after cervical vagotomy.
3. Vasopressin could not be detected in the blood during urinary bladder distension before or after section of the cervical vagus nerves. Cervical vagotomy itself resulted in a. transient release of vasopressin.
4. A prostaglandin-like material was sometimes detected in the circulation during or immediately after urinary bladder distension. The origin of this material was not determined; it was detected in arterial and venous blood.
5. Urine flow consistently decreased during urinary bladder distension. This decrease was due neither to circulating catecholamines nor to vasopressin; it was probably caused by renal sympathetic activity. Cervical vagotomy did not abolish the antidiuresis.