In order to determine the relationship between blood pressure and arterial blood flow velocity in various regional circulations, Doppler ultrasound measurements of blood flow velocity were recorded on the first day of postnatal life from the anterior cerebral (ACA), superior mesenteric, coeliac axis and left renal artery. In 34 ventilated very low birth weight (VLBW) infants, results were correlated with arterial blood pressure and blood gases in a multiple regression model. ACA velocity was correlated with blood pressure (r = 0.70) and PaCO2 (r = 0.64), but there was no relationship between blood pressure and velocity in the other arteries. Repeated measurements were performed at one week of age in 15 infants. Blood flow velocity in the cerebral and renal arteries was related to blood pressure; velocity in the cerebral arteries was inversely correlated with PaO2 and velocity in the coeliac and mesenteric arteries was positively correlated with PaO2. In VLBW infants on the first day of life, blood flow velocity is related to blood pressure in the cerebral circulation only.