Clinical and animal data show that there is a period of increased susceptibility to seizures during early postnatal development. The cause of this increased susceptibility is probably a combination of enhanced excitation and diminished inhibition throughout the brain, as well as developmental differences in subcortical circuits, such as the substantia nigra-mediated seizure-suppression system. Animal studies on the effect of prolonged epileptic seizures early in development indicate that even severe seizures produce little or no damage in the rat pup hippocampus. Several studies have demonstrated both qualitative and quantitative age-dependent differences in the effect of antiepileptic treatment. More studies are needed to determine the effects of long-term antiepileptic treatment.