The vasopressor and milk ejection activities were estimated in the supraoptic region of the hypothalamus of rats which had been hypophysectomized 30 min, 4 and 8 days earlier. Both pressor and milk ejection activities were significantly greater 4 days after hypophysectomy than those in sham-operated control rats. Eight days after hypophysectomy, pressor activity decreased below control values but milk ejection activity was still significantly raised. Increased biological activity after hypophysectomy is thus associated with the previously observed increase in neurosecretory material within the supraoptic nucleus so these results provide additional evidence that neurosecretory material represents stored hormone. Milk ejection activity increased relatively more than pressor activity which supports the suggestion that the final stage in the formation of neurohypophysial hormones occurs as they pass from the hypothalamus to the neural lobe. If this final maturation normally occurs more slowly for oxytocin than for vasopressin, the obstruction to the flow of hormones down the neural stalk caused by hypophysectomy would result in a greater increase in milk ejection than pressor activity.