Lumbo-sacral spinal roots and the nerve to the medial gastrocnemius muscle (NMG) from normal and dystrophic mice were examined by quantitative ultrastructural techniques. It was demonstrated that, although many axons at this level were totally devoid of Schwann cells, totatl axonal numbers were approximately normal. Some axons in these roots were surrounded by Schwann cells but their myelin sheaths were abnormally thin. In addition, cells resembling oligodendrocytes were observed in the dorsal roots of the dystrophic mice. In contrast, Schwann cells and myelin sheaths were normal in the more peripherally situated NMG and the regenerative response of these nerves to crush injury was not significantly different from that of control nerves. Thus, the main abnormality of neural morphology in dystrophic mice is a localized absence of Schwann cells. Such a deficiency does not appear to have influenced axonal growth or the capacity of these axons to regenerate when crushed distally.