Simultaneous recordings of multiple single unit activity in both cerebral and cerebellar cortex, cortical EEG, and both nuchal and vibrissal EMG were obtained in nine unrestrained rats. Putative Purkinje cells of the deep vermal cerebellar cortex exhibited rhythmic discharge of simple spikes with extremely low variability in interspike intervals for several hours. The highly rhythmic nature of spike discharge was remarkably stable across all states of sleep (both slow-wave and rapid eye movement sleep) and wake including quiet waking, grooming, eating, running in a familiar environment, and exploring a novel environment. The frequencies at which oscillatory discharges took place varied, among different cells, between 16 and 142 Hz; however, 75% of the recorded cells discharged at frequencies between 20 and 50 Hz. From recordings in which two to four such cells were recorded simultaneously, evidence was found for multiple cells firing at the same frequency as well as for multiple cells firing at different frequencies. The precise timing of spike discharge in these cells makes them potential candidates to participate in timing functions thought to depend on the cerebellum