The action of lateral habenula (LH) stimulation on focal epileptiform activity in the hippocampus was studied. Local microinjection of sodium penicillin induced a steady interictal activity in the dorsal hippocampus. Low frequency electrical stimulation of the habenula caused a marked enhancement of spike activity in both frequency and amplitude. The effect was blocked by intraperitoneally injected methysergide. The facilitatory influence of the habenula on hippocampal activity might be due to a disinhibitory mechanism. The results are regarded as suggesting that the habenula may be a relay station between the basal ganglia and the hippocampal formation. LH as well as basal ganglia might modulate hippocampal excitability, exerting a control on the genesis and diffusion of abnormal activities.