Limbic evoked potentials (LEPs) were recorded from the hippocampi of three epilepsy surgery patients with unilateral medial temporal lobe seizure onset. In each patient, stable large amplitude LEPs which displayed polarity inversion across successive levels of hippocampus were present unilaterally, but were absent or rudimentary in the hippocampus corresponding to the electrographically proven seizure onset. After administration of scopolamine, LEPs in the "normal" hippocampi were markedly altered with slowing of the individual components and increased trial to trial variability. In addition, two patients developed reliable, high amplitude, polarity inverting LEPs on the side of seizure onset which had either absent or rudimentary LEPs in the baseline state. The results suggest that LEPs are under cholinergic modulation.