The functional role of the cerebellum in voluntary movement was investigated by local cooling of the cerebellar nuclei in three Japanese monkeys which performed hand movement tasks in response to visual stimuli. We implanted electrodes in various areas of the cerebral hemispheres to record field potentials in the cortex, and examined effects of the cooling upon the movement and field potentials. Cooling of the dentate nucleus ipsilateral to the moving hand reversibly increased the reaction time and reduced the size of surface-negative, depth-positive (s-N, d-P) field potential in the motor cortex contralateral to the hand. The potential preceded the movement by an almost constant time of about 100 ms in the normal condition. The cooling remarkably prolonged and deviated the time. By shifting the cooling probe to different distances from the nucleus, we noted various decreases of the cooling effect. We also found a close correlation between the size of the s-N, d-P potential and the reaction time, i.e., when the potential was small, the reaction time was long. These findings support the following ideas; the motor command for this task comes to the motor cortex through the cerebello-thalamo-cortical pathway which includes the dentate nucleus, and produces the s-N, d-P potential as EPSP currents in pyramidal neurons in the motor cortex. We also studied self-paced movement task. In some cases, the cooling reduced the size of readiness potential in the motor cortex.