AbstractDiffusional anisotropy of water protons, induced by nonrandom, directional barriers which hinder or retard water motion, is measurable by MRI. Faster water diffusion was observed when the diffusion‐sensitizing gradient direction paralleled the long axes of white matter tracts, indicative of fewer bamers to water motion. Diffusion perpendicular to this 3xis was as much as four times slower. Anisotropy was seen pre‐ and postmortem in all axial, sagittal, and coronal planes, with and without cardiac gating. Ordering has also been observed in feline optic nerve and in human peripheral nerves. Utilization of this technique can greatly improve understanding and assessment of demyelinating disorders, of white matter infarcts and neoplasms, and of neonatal brain and spinal cord development. © 1991 Academic Press, Inc.