Systolic blood pressure in the arm was measured in infants at the ages of 4 to 6 days and 5 to 7 weeks by the Doppler ultrasound technique. At the age of 4 to 6 days the mean blood pressure (± SE of mean) in 469 sleeping infants was 70.7 ± 0.3 mm Hg, rising at 5 to 7 weeks to 89.7 ± 0.9 mm Hg (in 144 infants). In 252 infants awake at 5 to 7 weeks blood pressure was 96.8 ± 0.6 mm Hg. In 391 infants in whom measurements were made on both occasions blood pressure at 4 to 6 days was significantly related to blood pressure at 5 to 7 weeks. Thus those infants with relatively high blood pressures at 4 to 6 days showed a weak tendency to have relatively high blood pressures at 5 to 7 weeks. If this trend continues with age it would suggest that the tendency to develop hypertension may already be demonstrable at the age of 4 to 6 days.