The numbers of myelinated and non-myelinated nerve fibres in the spinal roots of one strain of mouse have been counted from electron micrographs and the accuracy of counting procedures assessed. Non-myelinated nerve fibres are present in the ventral roots at all levels though the largest number are found inthe thoracic and L1, L5, L6 and S1 ventral roots. The number is very small in L2, L3, and L4 ventral roots which are those supplying the hind limb. Fibre diameter histograms were obtained from measurements of cross sectional area for myelinated and non-myelinated fibres in L1 and L4 ventral roots. The relationship of myelin thickness to axon diameter shows that to a near approximation the myelin thickness is 1/5 to 1/4 the axon diameter. A measure of circularity was calculated, the ratio of the measured cross-sectional area to the area calculated from the measured perimeter assuming the fibre to be circular, and plotted against the axon perimeter fo L1 and L4 ventral root myelinated fibres, This showed a tendency for larger fibres to be less circular than smaller fibres.