Cerebral blood flow velocity was assessed during infusion of indomethacin over 30 minutes. Eleven preterm infants with symptomatic patent ductus arteriosus were studied on 12 occasions. Indomethacin infusion was associated with a significant reduction in time averaged mean velocity (TAMV), peak systolic velocity (PSV), and end diastolic velocity in both the anterior cerebral artery and middle cerebral artery. The fall in the TAMV and PSV was gradual with maximal change 30-40 minutes after the start of the infusion. It was concluded that administration of indomethacin by slow infusion produces haemodynamic alterations to the cerebral circulation comparable in magnitude with changes described with bolus administration. Indomethacin remains a useful and effective treatment for patent ductus arteriosus in preterm infants, but should continue to be used with caution.