Emergency ejection of an air crew member from military aircraft in flight places dangerously large vertical acceleration forces on the body of the member. The additional mass on the head due to Night Vision Goggles and Helmet Mounted Displays increases the vulnerability of the head/neck system to injury. To eliminate the need for human testing, computer simulations of biodynamic head and neck system response to large vertical accelerations have been produced. A head/neck characteristic was developed which included the rotation and axial deformation properties of the cervical spine. The characteristic consisted of three rigid segments representing the head, neck, and upper torso, a ball-and socket joint representing the head/neck articulation, and a slip joint representing the neck/torso articulation. The model was exercised using the Articulated Total Body Model developed by Calspan Corporation and Armstrong Laboratory. The model parameters were determined using human vertical deceleration test data acquired at Armstrong Laboratory. Simulations of human biodynamic response to ejection acceleration show the proposed head/neck characteristic to produce improved correlation with human biodynamic response to 10 Gz acceleration when compared to previous rigid body models of the human head/neck system.