Estimations of urine volumes and inulin U/P ratios in newborn rats after the administration of 4·5 ml. water and 10 mU. vasopressin/100 g. body weight failed to show an antidiuretic response comparable to that produced in adult controls. When body surface was chosen as the basis of dosage, relatively larger amounts of water were given to newborn rats. These produced an increase in the rate of urine flow (which, however, still remained well below the adult level), but an inhibitory effect of vasopressin again could not be demonstrated. Experiments on older rats (up to 4 weeks of age) showed that an antidiuretic effect of vasopressin comparable to that of adults develops only after several weeks of postnatal life, approximately at the same time as a full diuretic response to water could be obtained. When 10 mU. vasopressin/100 g. body weight was injected both adult and newborn rats excreted about 10% of this dose. It is suggested that vasopressin acts less effectively in newborn rats because for some time after birth the kidney is less responsive to the posterior pituitary antidiuretic hormone than in adults.