SUMMARY Cerebrospinal fluid (CSF) samples obtained by cisternal puncture from rabbits within 10 min. after intravenous pentobarbitone anaesthesia contained the equivalent of 57 ± 5·4 μ-u. arginine vasopressin/ml. Treatment with thioglycollate and trypsin abolished the antidiuretic activity. Stimulation of the central end of a severed vagus raised the concentration of antidiuretic activity in the CSF significantly. After the intravenous injection of vasopressin, antidiuretic activity measured in blood and CSF rose quickly, but the levels attained in the CSF were always lower than those in blood. When CSF samples were obtained with local anaesthesia no antidiuretic activity could be demonstrated; i.e. in terms of the sensitivity of the assay procedure used these samples contained less than the equivalent of 4·2 μ-u. vasopressin/ml. The significance of these findings is discussed in connexion with the morphological evidence suggesting the secretion of neurohypophysial hormones by neurosecretory neurones into the 3rd ventricle.