SUMMARY Synthetic oxytocin ('Syntocinon') and valyl oxytocin have been assayed for oxytocic and milk-ejection activities. In agreement with the previous results of others, valyl oxytocin exhibited about five times as much milk-ejection activity, per unit of oxytocic activity, as that shown by oxytocin. However, valyl oxytocin was no more active than synthetic oxytocin in retarding mammary involution in rats whose young were removed on the 4th day of lactation. This experiment, therefore, provided no evidence that the involution-retarding action of oxytocin is mediated by a direct effect on the myoepithelium of the mammary gland.