The apparent diffusion tensor (ADT) was measured in excised and fixed spinal cords from myelin‐deficient (md) rats and age‐matched controls. These data were used to obtain the principal diffusivities of the ADT, and also the scalar invariant parameters D̄ (averaged principal diffusivity) and Aσ (anisotropy index) for four white matter and two gray matter regions. The results for white matter regions showed that the principal diffusivities were significantly higher for md animals, and while the D̄ was increased in tissue from md animals, the Aσ was found to be decreased. Grey matter D̄ was measured to be between those of white matter from control and md animals, and the Aσ was much smaller than that of white matter from both sets of animals, indicating that diffusion in md white matter is more anisotropic than in gray matter. The results show that while myelination is not a prerequisite for diffusion anisotropy, it does influence the magnitude of the observed anisotropy. Magn Reson Med 45:191–195, 2001. © 2001 Wiley‐Liss, Inc.