A freshly dead Stumptail (Macaca speciosa) monkey brain hemisection model has been subjected to translation, pure rotation and a combination motion. Linear and angular head accelerations were measured as well as brain displacement relative to the skull and shear strain at several locations. Much higher than previously predicted shear strain was measured at acceleration levels which have been recorded during impacts which produced concussion in live monkeys. Pure rotation produced the highest, most diffuse and long lasting shear strain and brain displacement, while translation produced very low shear strain. Highest shear strain during rotation was recorded in the brainstem rather than on the periphery as many have predicted. Results suggest that the mechanism of brainstem injury, regardless of head motion, is due to shear caused by stretching of the cervical cord.