Newborn infants undergo major cardiovascular changes to accomplish the transition from fetal to newborn circulation. These include a drop in pulmonary vascular resistance and closure of the ductus arteriosus. We measured blood velocities by pulsed-Doppler ultrasound in the great vessels of normal newborns during this transition period. Our goals were to understand the timing of these changes and to assess cardiac function as measured by flow velocities. We concluded that shunting of blood through the ductus arteriosus continues for 1-2 days after birth without apparent hemodynamic consequences, cardiac function, as assessed by the Doppler technique, remains remarkably stable during this time period, the timing of ductal closure is not affected by the mode of delivery, and Doppler ultrasound is a portable, reproducible method which is useful in assessing the cardiovascular system of the neonate.