Cerebral artery Doppler ultrasound recordings, and intra-arterial pressure measurements have been made from 27 very low birth weight infants during the first week of life. These data were used to calculate Pourcelot's resistance index (RI), mean blood flow velocity, and resistance-area product (RAP), in both the anterior and middle cerebral arteries. There was a good degree of correlation between recordings from different sites on the same occasion, but absolute values were site dependent. Over the first three days of life there was a significant increase in blood flow velocity in each of the vessels, and a significant fall in RI and RAP.