Acceleration-time data in 25 squirrel monkeys subjected to controlled sagittal plane head motions are presented. In 12 of the 25 animals subjected to pure translation of the head at peak positive g levels ranging between 665-1230 g (6-8 ms duration), cerebral concussion was not obtainable. In contrast, 13 of the animals subjected to head rotations at peak positive tangential (at c.g.) g levels ranging between 348-1025 g (5.5-8 ms duration) were all concussed. Visible brain lesions were noted in both translated and rotated groups but with a greater frequency and severity after rotation. An analysis of the lesions produced in both groups is presented, along with our preliminary data on the use of the evoked somatosensory response as an objective, quantifiable index for the onset and severity of brain damage in head injury.