In order to investigate, during activation testing, the interictal cortical cerebral blood flows (rCBF) of epileptic patients suffering from complex partial seizures, 40 epileptic patients (divided into "lesional", this is, with abnormal CT findings, and "non lesional", that is, with normal CT findings) were submitted to rCBF measurements with the 133 xenon intravenous technique, at rest and during intermittent light stimulation (ILS). The findings compared with normal volunteers seem to demonstrate that, during ILS, (1) in non lesional patients, the suspected epileptic focus shows a significant rCBF increase (2) in lesional patients, the significant rCBF increases were not in the region of the suspected epileptic focus but in adjacent or in contralateral ones. It was concluded that activation interictal rCBF measurements are more useful than resting ones for the determination of epileptic foci when CT findings are normal and that the nature of the epileptic focus influences markedly the interhemispheric activation pattern.