The flexion of the cranial base and ontogenic migration of the foramen magnum in Macaca mulatto were investigated. Longitudinal cephalometric studies were carried out in infant and juvenile groups. Significant ontogenie changes of the cranial base were observed only in juvenile age group. The infant group did not exhibit any changes of the cranial base angle or migration of the foramen magnum. The diameter of the latter in infants exhibited small but statistically significant changes.