The rotational and translational rigid body motions of the head after impact were evaluated by high-speed cinematography in Rhesus monkeys with and without a cervical collar. When a collar was worn, animals displayed increased tolerance to occipital impact for the onset of cerebral concussion. Although head rotations were reduced in this nonconcussed protected group, translational motion of the head exceeded that attained by concussed monkeys not wearing collars but struck at equivalent impulse levels. These data emphasize the inadequacy of current head impact tolerance criteria which relate the occurrence of brain injury to translational head motions.