Near infrared spectroscopy was used to investigate the effects of intravenously administered indomethacin (0.1-0.2 mg/kg) on cerebral haemodynamics and oxygen delivery in 13 very preterm infants treated for patent ductus arteriosus. 7 infants received indomethacin by rapid injection (30 s) and 6 by slow infusion (20-30 min). In all the infants cerebral blood flow, oxygen delivery, blood volume, and the reactivity of blood volume to changes in arterial carbon dioxide tension fell sharply after indomethacin. There were no differences in the effects of rapid and slow infusion. These falls in cerebral oxygen delivery and the disruption of cerebrovascular control might compromise cellular oxygen availability, particularly in regions of the brain where the arterial supply is precarious. Care should be taken to ensure that oxygen delivery is optimum before the administration of indomethacin to preterm infants.