1. The specific radioactivities of isotopically pure oxytocin and vasopressin prepared from the neural lobe of the pituitary gland have been measured at various times after an intracisternal injection of [(3)H]tyrosine.2. Radioactive hormone began to appear in the gland 1-2 hr after the injection which suggests an intra-axonal transport velocity of 1-2 mm/hr.3. From 7 days onwards the specific radioactivity of each hormone declined exponentially with the same rate constant and a half-life of about 13 days.4. If the decline in radioactivity can be equated with the release of the hormones, the rates of secretion for the male rat in water balance are 18.7 m-u./day for oxytocin and 28.9 m-u./day for vasopressin.5. Calculation from the secretion rates gave steady-state plasma concentrations of about 3 muu./ml. for each hormone.