Blood velocity in the internal carotid artery (ICA) and in the descending thoracic aorta (DAo) was investigated used Duplex‐Doppler ultrasound in 14 infants of<30 weeks gestation, treated prophylactically with surfactant, and in 11 comparable infants with relatively mature lungs who served as controls. After surfactant administration, blood gases, pH or FiO2were not different between the groups. Temporal mean blood velocity in the ICA was used as a relative measure of cerebral flow (TMFV‐cer), and its coefficient of variation (CV‐cer) was used to assess fluctuations in cerebral blood velocity. The pulsatility index (PI) in the ICA (PI‐cer) and DAo (PI‐DAo) was used to estimate if a left‐to‐right shunt was present. During surfactant instillation TMFV‐cer was abnormally low and CV‐cer indicated a fluctuating cerebral blood velocity. At 10 min after surfactant administration, TMFV‐cer of the treated infants was higher compared to the controls, while CV‐cer was stable in both groups. PI‐cer and PI‐DAo were abnormally high during the first hour of life after surfactant treatment, suggesting a left‐to‐right shunt without, however, clinical signs of a hemodynamically important ductus arteriosus. We suggest that cerebral perfusion is affected during and at 10 min after surfactant instillation. Left‐to‐right shunting appears to be a common eve