Studies on the side effects of diazepam on the particularly vulnerable brain of preterm infants have not been done so far. We studied changes of blood flow velocity in the anterior, basilar and internal carotid artery by Doppler technique in 11 preterm infants less than 1,500 g who were given 0.5 mg/kg i.v. diazepam. The flow velocities in the internal carotid and basilar artery did not change significantly and values for carbon dioxide tension, mean arterial blood pressure and HF remained stable. There was a marked increase of flow velocity in the anterior cerebral artery. We conclude that diazepam in this dosage does not cause dangerous haemodynamic changes in the premature brain.