Traumatic damage to the brainstem occurs frequently when the brain skull complex experiences injurious loading especially during those traumatic situations that produce diffuse axonal injury (DAI). DAI has been shown to be dependent on load direction and correlated with regional tissue deformation in response to rotational inertial loads. Possible mechanisms for the selective vulnerability of the brainstem are (1) the geometry of the central nervous system is responsible for producing high tissue strains in these regions, (2) regional differences in overall material stiffness result in larger deformations at these sites, and (3) the anisotropic mechanical properties of these regions lead to a sensitivity to the rotational load direction and magnitude. This paper investigates the latter two hypotheses by performing oscillatory shear tests on adult porcine brainstem in three mutually perpendicular directions. The complex shear moduli were calculated over a range of frequencies (20-200 Hz), for three levels of peak engineering strain (2.5%, 5.0%, and 7.5%). The directional data demonstrated that the brainstem exhibits significant transversely isotropic behavior. Both components of the complex modulus in which the axonal fibers are oriented parallel to the plane of shear but transverse to the shear direction were significantly higher than those of the other two, mutually indistinguishable test cases across the range of strains tested. By comparison with similar tests on cerebral tissue, these data demonstrated that the brainstem displays a stiffer biomechanical response. These differences were present for both components of the complex shear modulus and were greater as the magnitude of the applied strain increased. The regional stiffness and anisotropic response of the brainstem coupled with its location as a narrow bridge between CNS regions interact to result in the selective vulnerability of this region in rotational loading.