Clinical studies indicate that early epilepsy after head injury may be associated with some transient and reversible pathophysical processes of the brain. It has been proposed that epileptogenesis in the neocortex and hippocampus may be related to potassium ion accumulation in extracellular spaces. To investigate this hypothesis, we measured [K+]0 using potassium-sensitive microelectrodes in the sensorimotor cortex of cats during early seizures induced by trauma. The [K+]0 increases associated with seizure activity ranged from 14.6 to 25.1 mM, and these were significantly higher than those unassociated with spikes or seizure discharges. Moreover, high K+ solutions (15 mM or more) directly applied to the cortex produced spiking and seizures. These results seem to support the hypothesis that accumulation of [K+]0 is related to development of early epilepsy.