Indomethacin decreases cerebral blood flow velocity and blood flow in the preterm infant. The duration of this negative effect has not been established. Cerebral blood flow velocity was evaluated in 24 preterm infants with symptomatic patent ductus arteriosus before and during the first 12 hours after a single intravenous dose of indomethacin, 0.1 mg/kg. Cerebral blood flow velocity was estimated by serial Doppler investigations of the anterior cerebral arteries. Indomethacin administration led to an instantaneous decrease of peak systolic flow velocity, temporal mean flow velocity, and end-diastolic flow velocity of the anterior cerebral arteries in all infants, which was maximal between 2 and 40 minutes after indomethacin administration and was followed by a more sustained recovery of all velocities to baseline values. Temporal mean flow velocity was not different from pre-indomethacin values at 3 hours after the administration. It is concluded that indomethacin can impact the cerebral circulation of the preterm infant for at least 2 hours. This may have consequences in preterm infants with unstable hemodynamics and pulmonary function.