Kinetics and kinematics of the quadrupedal walking of Ateles and Macaca were studied, and the results were compared with those of vertical climbing. Macaca seems to use similar strategies for locomotion in climbing and walking horizontally, whereas Ateles apparently changes strategies. Next, volumetric comparisons of three cerebellar nuclei (M, I, L), the ventral pons (VP), the inferior olivary principal (Pr) and accessory (Ac) nuclei, and the vestibular nuclear complex (VES) were made by using size indices of each nucleus. Higher values are recognized clearly in the medial zone group (M, Ac, VES) of Macaca, whereas almost similar values are shown to exist in the lateral zone group (L, Pr, VP) of both species. Relationships between the kinesiological results and the development of the cerebellar system are discussed.