The authors provide evidence that four mesencephalic nuclei, two of which are cholinergic and two of which are catecholaminergic, serve as oscillators capable of modulating a number of interrelated rhythmic functions. These include sleep-wake cycles, locomotion, blood pressure, respiratory rate, heart rate, mastication, micturition, and saccadic eye movements. The P1 auditory evoked potential is generated by ascendiung cholinergic projections from one of these nuclei, the pedunculopontine nucleus, and thus serves as a clinical tool to monitor the integrity of this system.