The aortic pressure was measured via an umbilical artery in 74 normal full-term and 26 premature infants. The ages ranged from 1 to 77 hours and the weights from 1.2 to 4.5 kg. The pressure was lower in the premature than in the full-term group and there was a positive correlation with age in both groups and with weight in the premature group. Vasomotor reactivity was appraised by use of the cold pressor test and postural tilting. The results indicate that the vasomotor regulatory mechanisms are present and functional at birth in premature as well as in full-term infants.