Repeated electrode penetration of the dentate and interpositus nuclei in a rhesus monkey transformed an 11–13 Hz physiologic tremor into a much larger action tremor at 5–7 Hz. This tremor was associated with muscle spindle spike train modulation and reflexly evoked tremor modulation of interpositus and motor cortex neurons as well as electromyogram (EMG). No tremor modulation was observed in spike trains recorded from dentate. The timing relationships of the spindle, EMG, and interpositus tremor discharges suggest that the interpositus plays a direct role in tremor suppression. Dentate, by contrast, may function indirectly by setting optimal transcortical long loop reflex dynamics concerned with intended voluntary movement.