Movements elicited by the stimulation of the cerebellar nuclei were studied in alert baboons chronically prepared. The motor responses were filmed and recorded in eight muscles through chronically implanted electrodes. Two types of motor effects were observed: (1) Simple movements that concerned the unidirectional displacement of a limb segment. (2) Complex movements that involved distinct and frequently noncontiguous muscles were stereotyped and could not be dissociated. These movements are defined as motor synergies. Electromyographic study allowed us to investigate the motor response latencies and the modality of cerebellar control on musculature. Simple movements were due to the activation of muscles within the involved segment in addition to the co-contraction of muscles of a nearby segment. Thus they could be due to a cerebellar control over muscular synergies. Complex movements would correspond to the simultaneous activation of distinct muscular groups and could also be the outcome of a cerebellar control on motor synergies. Thus the effects of the interposed nucleus concern preferably flexor muscles whereas the effects of the dentate nucleus appear to be equally distributed among flexor and extensor muscles. Somatotopic motor localization were evidenced both in the interposed and dentate nuclei: there are somatotopic relations between every region of the interposed nucleus and musculature. As regards the dentate nucleus, two subdivisions were distinguished according to the complexity of elicited motor effects: (A) an antero-medial region from which motor synergies can be elicited. (B) a postero-lateral region giving rise to simple movements, mainly hand movements.