Cerebral hemodynamics were studied in 7 term newborn lambs and 10 preterm infants with patent ductus arteriosus. Intracranial blood flow velocities were measured using a pulsed-wave Doppler velocimeter applied through the fontanel. Intravenous injection of indomethacin (0.2 mg.kg-1) reduced intracranial blood flow velocities in both lambs and infants. The CO2 reactivity, however, did not change in the lambs. Indomethacin administration is associated with reduced cerebral blood flow velocities but the pharmacological mechanism is unclear.