Measurements of renal artery blood flow velocity have been previously reported during prenatal life and during the first month of life in neonates. Since renal flow velocity increases during the first 3 days of postnatal age we determined, during the fourth day of life, the changes of renal blood flow velocity in relation to gestational age and birth weight in 109 healthy preterm and term infants with gestational age between 26 and 42 weeks and birth weight between 750 and 3,730 g. Renal arteries were insonated about 3 mm from the abdominal aorta, by placement of a microconvex 7-MHz transducer below the costal margin in the dorsolateral area of the right or left flank. Mean flow velocity significantly increased with advancing of gestational age and birth weight. This study provides normative data for renal artery blood flow velocity in healthy preterm and term infants during the fourth day of life.