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Experimental tests using porcine brainstem samples were performed on a custom designed stress relaxation shear device. Tests were performed dynamically at strain rates >1 s−1, to three levels of peak strain (2.5%-7.5%). The directional dependence of the material properties was investigated by shearing both parallel and transverse to the predominant direction of the axonal fibers. Quasi-linear viscoelastic theory was used to describe the reduced relaxation response and the instantaneous elastic function. The time constants of the reduced relaxation function demonstrate no directional dependence; however, the relative magnitude of the exponential functions and the parameter representing the final limiting value are significantly different for each direction. The elastic function qualitatively demonstrates a dependence on direction. These results suggest that the brainstem is an anisotropic material.