The response of the head to impact in the posterior-to-anterior direction was investigated with live anesthetized and post-mortem primates.* The purpose of the project was to relate animal test results to previous head impact tests conducted with cadavers (reported at the 21st Stapp Car Crash Conference (1),** and to study the differences between the living and post-mortem state in terms of mechanical response.The three-dimensional motion of the head, during and after impact, was derived from experimental measurements and expressed as kinematic quantities in various reference frames. Comparison of kinematic quantities between subjects is normally done by referring the results to a standard anatomical reference frame, or to a predefined laboratory reference frame. This paper uses an additional method for describing the kinematics of head motion through the use of Frenet-Serret frame fields.The experimental technique used a nine-accelerometer system, mounted rigidly to the head, to measure head motions. Additional measurements included impact force, epidural pressure, and strains in the skull bone. High-speed cineradiography (1000 frames/second) was used during the impact. A total of seven animals were tested in the project, five post-mortem and two live.The results of the tests are presented to demonstrate the similarities and differences found between animal and human cadaver subjects and between living and post-mortem subjects. The effects of the following factors are discussed: 1.Relative magnitudes of brain mass, skull mass and external soft tissue mass. 2.Head surface geometry at the impact site and mass distribution. 3.Differences in epidural pressures and in head trajectories.