Mechanical impacts were delivered by an air propelled striker to the front, side, rear and top of rigid protective caps worn by six anesthetized monkeys. These tests were to produce reversible concussion and to determine differences in tolerance to concussion among the four impact sites. Striker force and cap accelerations were measures of the impact severity and animal blood pressure, respiration and ECG changes were measures of the physiological effects.By distributing the blow with a protective cap, allowing free head movement after impact, skull fracture was eliminated and simple reversible concussion could be produced without symptoms of residual neurological deficit. Higher linear and angular accelerations produced longer periods of unconsciousness (more than 3 times) on the side than at any of the other locations. It is hypothesized that the decrease in concussion tolerance accompanied by higher accelerations for side impacts may be the result of lower mechanical impedance due to the oval shape of the animal head.