The relationship between changes in EEG and the extracellular potassium concentration (K+e) in brain cortex was studied in rats during petylenetetrazol seizures and during respiratory arrest. During seizures an increase of K+e occurred subsequent to the electrical discharges, while during postictal depression of EEG normal levels of K+e were measured. During respiratory arrest K+e rose considerably one minute after flattering of EEG. These findings are against a concept of K+e determining the changes of the EEG pattern. It is hypothetized that during seizures the increased K+e may play an important role in the coupling of neuronal activity to cerebral blood flow and metabolism.