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Cholinergic stimulation of the hippocampal formation results in excitation and/or seizure. We report here, using whole-cell patch-clamp techniques in the hippocampal slice (34–35°C), a cholinergic-dependent slow afterdepolarization (sADP) and long-lasting plateau potential (PP). In the presence of 20 μm carbachol, action potential firing evoked by weak intracellular current injection elicited an sADP that lasted several seconds. Increased spike firing evoked by stronger depolarizing stimuli resulted in long-duration PPs maintained close to −20 mV. Removal of either Na⁺ or Ca²⁺ from the external media, intracellular Ca²⁺([Ca²⁺]_i) chelation with 10 mmbis(2-aminophenoxy)ethane-N,N,N′,N′-tetra-acetic acid, or the addition of 100 μmCd²⁺ to the perfusate abolished both the sADP and PP. The sADP was depressed and the PP was abolished by either 10 μm nimodipine or 1 μmω-conotoxin, whereas 1.2 μm …