1. The milk-ejection activity of highly purified oxytocic and vasopressor-antidiuretic (ADH) polypeptides was determined on the intact mammary gland of the lactating rabbit. 2. When assayed in this way, 100 units of vasopressin-ADH had an activity represented by 17 units of oxytocin. 3. The lactating rabbit mammary gland is more sensitive to oxytocin than the rat uterus, and appears to be the best means at present available for assessing milk-ejection activity.