Study of immature rabbit hippocampus, using the in vitro slice preparation, has revealed seizure-like spreading depression (SD) episodes in tissue from 8-12-day-old animals. These SDs occur both spontaneously and in response to stimulation, and are seen in both extracellular and intracellular recordings. At the cellular level, SDs are similar to epileptiform ictal phenomena in their onset bursts of action potentials, prolonged membrane depolarizations, and afterdischarge bursts. Glial recordings indicate that a large rise in [K+]0 occurs during these SD episodes. Low chloride concentration in the bathing medium facilitates SD occurrence. The immature CA1 region, where inhibition is slow to develop, is more susceptible to SDs than the CA3 region, where inhibitory post-synaptic potentials are potent early in development. These observations suggest that the slice preparation of immature rabbit hippocampus may provide a useful model in which to study epileptiform mechanisms, such as inhibitory efficacy and extracellular potassium clearance, which might be responsible for ictal onset and control.