Cross-sections of 17 sciatic nerves were examined by an image analyzer as to sum of fibre carrying cross-sectional areas and by light-microscopic visual area sampling as to myelinated and unmyelinated fibre densities. Intra-perineural cross-sectional areas range from 3.5 ×106 to 7.6 ×106 µm2 with a mean of 5.8 ×106 in the 12 male subjects and 3.4 ×106 to 4.0 ×106, x = 3.7 ×106 µm2 in 4 mature females. Estimates of total fibre content range from 6.1 × 104 to 1.6 × 105 in the males and 6.1 × 104 to 9.2 × 104 in these female subjects. Variations in axon number totals and in ratios of myelinated to unmyelinated fibre densities are not age correlated. Variations also occur in the diameter distributions of myelinated fibres and in ratios of small to large diameter myelin