The relationship of neonatal seizures to changes in the cerebral circulation was studied in 12 premature newborn infants. The objectives of the study were to determine whether important alterations in cerebral hemodynamics occur with neonatal seizures and whether such alterations relate to systemic hemodynamic events. Blood flow velocity in the anterior cerebral arteries was measured by a transcutaneous Doppler technique. A marked increase in cerebral blood flow velocity was documented with seizures in every patient. The prominent changes in the cerebral circulation occurred despite the fact that 10 of the 12 infants had only subtle seizures and all 12 were receiving mechanical ventilation at the time of the seizures. Accompanying the increase in cerebral flow velocity was a marked increase in intracranial pressure. The cerebral hemodynamic changes appeared to reflect directly changes in systemic hemodynamic events, that is, a marked increase in blood pressure at the time of seizures. The increase in cerebral blood flow velocity with seizures, an apparent adaptive physiologic response in older individuals, may be maladaptive in the newborn infant with certain vulnerable capillary beds, such as the germinal matrix in the premature infant or the margins of an infarct in the asphyxiated infant.