The study of epilepsy serves to emphasize the importance of integrating basic research with clinical data. In this selective review, the results of recent experiments using intracellular recording techniques, ion-specific microelectrodes, and a methodology suitable for studying mammalian cortex in vitro are discussed in terms of cellular phenomena which underlie clinical observations made in epileptic patients. This information has begun to clarify: (1) the changes in neuronal behavior associated with interictal and ictal events; (2) alterations in local ionic microenvironment which occur during focal epileptogenesis; (3) intrinsic control mechanisms which serve to restrict seizure spread; (4) neuronal characteristics which account for the differences in seizure patterns seen in infants and adults; and (5) the possible long-term consequences of recurrent local neuronal hyperexcitability.