I propose that hyperactivity (or increased numbers) of neurons in the brainstem reticular core promote hallucinations by excessive excitation, coupled with disinhibition, of target thalamocortical neurons. This idea stems from animal experiments using kainate-induced overexcitation of midbrain reticular neurons leading to hallucinatory-type behavior during wakefulness, as well as from data in schizophrenic patients showing an increased number of neurons in mesopontine cholinergic neurons.