Cerebrovascular responses were studied in preterm infants by Doppler ultrasound and cerebral electrical impedance for one hour after intravenous indomethacin infusion for patent ductus arteriosus. Indomethacin in a dose of 0.2 mg/kg body weight was infused over five minutes in one group of infants (20 doses) and over 20 minutes in a second group of infants (16 doses). There were no significant differences between the two groups in birth weight (mean = 1068 g, range 569-1950), gestational age (mean 28.4 weeks, range 25-31), or postnatal age (mean 18.1 days, range 6-42). There was a significant reduction in both the Doppler mean flow velocity in the anterior cerebral artery (mean (SE) -20 (4.2)%) and peak amplitude of cerebral electrical impedance (-26 (3.9)%) within two minutes of starting the indomethacin infusion over five minutes. There was no significant change after the infusion over 20 minutes. There were no significant changes in blood pressure or carbon dioxide tensions after infusion at either rate. The results suggest that infusion of indomethacin over five minutes caused a potentially deleterious reduction in cerebral blood flow. No such reduction occurred when it was infused over 20 minutes.