The effect of magnesium-free medium on the electrical activity in CA3 of the rat hippocampal slice was examined. Magnesium removal resulted in the development of spontaneous and triggered interictal-like bursting, followed by spontaneous ictal-like events and finally periodic clustered bursts. The ictal-like events consisted of a tonic firing phase and a phase of clustered burst discharges resembling the tonic and clonic phases of seizures. The return to normal medium resulted in spontaneous and triggered interictal-like bursts.