In 51 children with different types of epilepsy, blood flow velocities in the middle cerebral artery were recorded continuously by transcranial Doppler sonography during a standard electroencephalogram of 30 min duration. In 16 children 33 epileptic seizures were recorded. During tonic seizures, the mean flow velocity increased to a maximum of 133%-191% (median 160%) of the baseline values. Tonic-clonic seizures were also accompanied by a velocity increase. During absence seizures the mean flow velocity decreased to a minimum of 46%-82% (median 71%) of the baseline values. Changes in cerebral metabolism and arterial blood pressure in the presence of disturbed autoregulation are thought to be factors causing these alterations. No alteration of the flow velocities occurred in cases of petit-mal status, electrical status epilepticus and in 35 children with generalized epileptic discharges of up to 5 s duration without clinical manifestations.