We have used two main techniques to investigate the effect of surfactant therapy on the cerebral circulation in 34 babies. Twenty-one babies were studied using near infrared spectroscopy and 13 had cerebral blood flow measured using a xenon clearance technique. Other measures recorded included amplitude integrated EEG, mean arterial blood pressure and central venous or oesophageal pressure. There was a marked depression in EEG activity lasting for 10–20 min after surfactant administration. Cerebral blood volume and cerebral blood flow were maintained in most infants in spite of a fall in mean arterial blood pressure. We conclude that factors other than cerebral ischaemia cause the transient disturbance in electrophysiologic activity.